[Let's kill tonight.]

You should just talk to the doctor firstly about your period irregularity, and how you're concerned about your future. It doesn't necessarily mean you want to get pregnant now if you're talking about your current health.. I really wouldn't worry about it. You could even just say you were wondering how long after the pill it would take your period to go back to normal, and he may just prescribe you something to regulate your period a little better - if that didn't work, maybe then push some more and he would be able to look at fertility tests..
Also, maybe talk to your fiancee a little more about the matter.. even with fertility issues, there are still other methods to conceive a child