lack of a period

when I was 17 I took birth control that wasn't prescribed to me for 2 months and ever since it's either I have bled for 6 months straight non-stop or I don't bleed at all. I get random 2 minute long cramps near my pelvic/uterine area like every other week to every month. I have gone to ob/gyns and explained what is going on for them to say nothing, give me pap smears, and prescribe birth control (which has done nothing but make me cramp uncontrollably). I'm terrified that I'll never be able to have children. I just don't know what to do. :(