Am I pregnant?

Me and my boyfriend had sex on February 9th, when he came in my without my knowledge. (Very big fight) I've been on the pill for about 3 months now, and I missed the time to take it in February twice. So I took plan b, I read that it doesn't work on women over a certain weight, which I am. My period was 2 days late, but I got it on the 11th, and it lasted 7 days. I haven't had sex since then, but this month my period is 4 days late. I've taken 4 tests, all negative. Is it possible that the sperm still got there? Could I be pregnant? Please help