[Always *]

Have you taken a test? If you have gotten your period I'd say your fine but my friend actually didn't find out she was pregnant until she missed what should have been 2 periods because I guess in rare cases some woman experience implant bleeding (I believe this is the correct word). So if you believe you have cause for concern then taking a test might not be such a bad idea. But that being said, it is possible for periods to fluctuate, a period can be heavier or lighter or have different consistencies in the blood depending on what's going on. We all have the way our period "usually" is, but unless it's a dramatic change and you are experiencing a lot of discomfort in some other way you don't necessarily need to worry. But again, it is your call because it is your body and worst comes to worst you have spent a few bucks for "no reason" (though I personally think putting any concerns to rest is perfectly legitimate).
If you wish to take a test you can get the usual brands at the drug stores which I think are usually around $12 and most come in packs of 2. You can also buy them at dollar stores for much cheaper and they are just as effective as they are else where. I believe that taking it in the morning often gives the best results because that's when your hormones are at a certain level. But you can realistically take it at any time of the day.
If you are having sex I would strongly recommend going on the pill or some other form of contraceptive if you haven't done so already in addition to using condoms. You can never have to much protection. And I personally found for myself that using the pill helps me to worry less about my period and/or pregnancy because 1. I know when to expect my period because it's regulated by the pill and 2. I know that I am still protected from pregnancy if for some reason condoms fail. It also makes your period lighter if you are on a contraceptive. So if you have recently started taking the pill in the last few months this could just be the sign that your body is fully adjusting to the pill because your period won't necessarily lighten up immediately.