What Causes Irregularities In Periods?

Ever since I started my period (13) it had been irregular. I used to menstruate usually every 2-3 months, and very lightly. Last year, when my boyfriend and I started having sex, I started taking the pill. At that point, I was getting my periods every month, like clockwork, except that they would last around a week and a half, and they would occur twice a month. So, I stopped taking the pill. Up until last month, even after I had stopped taking the pill, my period was still coming once a month. However, this month I am late by approx. two weeks. I am wondering if this could just be my body reverting itself back to my regular cycle, or if there is a possibility that I'm pregnant. My boyfriend and I use condoms, but usually we start having sex without one, then when he gets close to climax, he puts on a condom. Some condoms have broken, and sometimes we progress too fast, and he climaxes before he puts a condom on. He always pulls out in this circumstance, but I realize that it's still possible that he didn't pull out fast enough. I have searched the internet and cannot find anything pertaining to my situation directly, and really need help!