HELP TTC

Hello girls I am trying to get pregnant, have been for a while now and need some advice. I am 21 years old and have a almost 3 year old girl. After I had her I wasn’t trying to have another baby but me and my boyfriend never use condoms, we been together for 7 years since he was 13!! Anyways.. after I had my daughter my body changed A LOT! In may of 2015 I had my daughter but that same month I stopped taking my prenatal pills and my doctor told me to continue taking them after I had my baby and I honestly never took them, do you think that has something to do with me not getting pregnant? I’ve been having unprotected sex after my daughter for 3 years now and still nothing.... there was a time where I didn’t even start my period for like half a year after I had my daughter then it turned to having a period every other month.. my doctor thinks it’s cuz I gained a lot of weight from my pregnancy but I don’t think it is... please has anyone else had this happen before and still gotten pregnant? If so, how long did it take?