[WashoutThePain]

Hopefully some of this will help I bolded a couple of the important parts. If you do end up having problems with erections with your boyfriend, once or twice is probably just a fluke, but if it becomes more, I encourage you to talk to him about it and see what's up. For the one or two times it happens though, just be sure not to act disappointed or upset. He's probably really embarrassed already, so comfort him and see what happens

From a reliable source (I think )

Causes
An erection requires the interaction of your brain, nerves, hormones, and blood vessels. Anything that interferes with the normal process can lead to a problem.
Common causes of erection problems include:
Diseases and conditions such as diabetes, high blood pressure, heart or thyroid conditions, poor blood flow, depression, or neurologic disorders (such as multiple sclerosis or Parkinson's disease)
Medications such as blood pressure medications (especially beta-blockers), heart medications (such as digoxin), some peptic ulcer medications, sleeping pills, and antidepressants
Nerve damage from prostate surgery
Nicotine, alcohol, or cocaine use
Poor communication with your partner
Repeated feelings of doubt and failure or negative communication that reinforce the erection problems
Spinal cord injury
Stress, fear, anxiety, or anger
Unrealistic sexual expectations, which make sex a task rather than a pleasure
Erection problems tend to become more common as you age, but they can affect men at any age and at any time in their lives. Physical causes are more common in older men, while psychological causes are more common in younger men.
Low levels of testosterone rarely lead to erection problems, but may reduce a man's sex drive.
Home Care
For many men, lifestyle changes can help:
Cut down on smoking, alcohol, and illegal drugs.
Get plenty of rest and take time to relax.
Exercise and eat a healthy diet to maintain good circulation.
Use safe sex practices, which reduces fear of HIV and STDs.
Talk openly to your partner about sex and your relationship. If you are unable to do this, counseling can help.
Couples who cannot talk to each other are likely to have problems with sexual intimacy. Men who have trouble communicating their feelings may find it difficult to share with their partner any anxieties about their sexual performance. In these circumstances, counseling can be very helpful for both you and your partner.

This is from Cosmopolitan (not entirely reliable, but seems to be spot on to me):

Men who are overweight, sedentary, or have other health issues are at high risk for erectile dysfunction. If your boyfriend doesn’t exercise or is above his ideal body weight, he may need to get active and make some healthy diet changes to boost his erection ability. Also, any health problems that interfere with blood flow — a key component of erectile ability — such as diabetes or heart disease, may also be the culprit. He should discuss the problem with his doctor and explore solutions.

Your boyfriend’s mental health plays an important role as well in his ability to stay erect. Depression, anxiety, and stress can all take a toll. Even conflicted feelings about the relationship, which is usually stressful, can make it difficult for him to maintain an erection. Medications to treat depression or anxiety, as well as alcohol and recreational drug use, can also interfere with erectile function.

Different Source (Not entirely reliable either, but from what I know, it's right):

This kind of erection failure is rather simple to explain: it is an anxiety-based response. Anxiety can effectively turn off the sexual response in young men because it is a product of the activity of the sympathetic nervous system, which opposes the sexual responses controlled by the parasympathetic nervous system. In effect, therefore, one can say that over-arousal is the cause of many young men's erection problems, though this refers more to emotional arousal than sexual arousal. Paradoxically, over-arousal can also cause premature ejaculation: the essence of both problems is that a man's normal sexual response has been over-ridden by his anxiety.

We discussed the effect of an over-sensitive penis in the debate on premature ejaculation. Can it also be a factor in erectile dysfunction? But first of all we need to define what we mean by an over-sensitive penis: in reality, of course, there is no such thing. It is a short hand term for a penis which sends nerve impulses to a man whose brain is already over-excited, too sexually aroused. As with premature ejaculation, corrective action requires some reduction in the sensitivity of the nervous system so that it can take more stimulation with no disruption of a man's sexual circuitry!

The irony here is that anxiety - fear, worry, call it what you will - about losing your erection actually stimulates a man's level of nervous system activity so that he is actually somewhat more likely to lose it! This means that relaxation, or at least a methodology that can help to reduce one's nervous and emotional arousal during sex, may be part of the answer. Obviously relaxation and hypnosis may be of help here; so is the experience which comes with age and having had a number of lovers.

Young men of course tend to be very aroused, and may in addition both fear discovery (especially in societies where pre-marital sex is frowned upon) and pregnancy. All of this can stimulate their nervous systems so that they are more likely to be sensitive to any suggestion by their lover that strikes at their sexual confidence: and makes them lose their erection. Fear and sex do not mix well!

A man who loses his erection may find that this happens instantly during sex: he cannot reduce his anxiety enough to reduce the stimulation he's receiving to a point where he no longer feels so aroused that he loses control of his sexual responses.