helppppp

so about a week ago I had sex without a condom Ive been on birth control since June 5th and it's now August. I've been having cramps and back pain Friday and yesterday but no period? just brown discharge. the guy didn't finish in me he pulled out about 3 minutes before and then finished else where. I don't think I'm pregnant but I'm really concerned about not having a period and today I start a new pack of pills. has this happened to anyone else?