I Had Unprotected Sex (first Time) Which Is Not The

I had unprotected sex (first time) which is not the smartest idea ever and it didn't last very long it was for about 4 minutes and he did not finish inside me I still have this very scary thought in the back of my head that there is a chance of someone getting pregnant from precum but I'm on birth control and I heard the birth control is not effective maybe after a month or 3 months and I'm on a low dosage of it its my first time of being on birth control and I have been having some brownish discharge and I have these cramps that I feel like I'm on my period but I'm only halfway through my pack of my birth control pills the discharge has cleared up after a couple days but now its like this really stringy like brown clots or something so please tell me what does this mean I'm very worried about this situation.