What is the possibility of getting pregnant while on the pill?

So I have been on the pill since about 3 years now and have never gotten pregnant in the past. I'll admit, sometimes I forget to take my pill and I don't always take it at the same time, but I try to take it around the same time, and when I forget I usually remember sometime when I wake up in the middle of the night or the next morning and take it. Last night, my boyfriend and I had sex and the condom broke and we didn't know and he came inside me. I have considered going out to get the Plan B pill but I don't want it to screw up my menstural cycle because it took about a year to find a birth control that works for me (I used to get 2 week - 1 month long periods). I am supposed to get my period this week, usually I get it Tuesday or Wednesday, and I feel like I'm having cramps. I'm 18 and still don't want to talk to my mom, even though I know she'll be understanding. I just need some answers. And is there a birth control thats better or worse? I take Yasmin birth control, if that helps.