Emma, I just wanted to tell you that it takes the average couple 8 months of actively trying before they conceive. So three months is not that long especially considering you were on "the shot" (depo, i'm assuming). Depo is notorious for screwing up your cycles and hormones, and it takes women months to get back to their normal cycles. So I wouldn't panic just yet. Most likely it will happen for you eventually.
That being said, if you don't feel like waiting around and are getting anxious, then there are some things you can do. You should probably see your doctor again (or a different doctor, if the one you have is not giving you any real answers) to
a)get a pre-pregnancy check up
b)get complete std testing (lots of stds don't have symptoms)
c)discuss how a previous std could affect your ability to get pregnant
also, you can start using "natural family planning" (which involves charting your fertility signs so that you can tell which day you are ovulating, and have sex on the few days before then to maximize your chances...Timing is everything in this game!). You can find tons of info on pregnancy forums about it.
Good luck!
Ps you should also start taking prenatal vitamins, because ideally you want to be taking them every day for 3 months before you get pregnant.