short brown period on placebos, pregnant?

Hi, im 17 and this is my 4the month of Beyaz. I have been late on a few pills (around 2 hours) but aside from the 3 late pills I take my birth control at the exact same time every night. My birth control had 4 placebo pills, and I usually start on the 3rd or 4the pill. Yesterday (the third pill) my period started but was very dark brown and red and only was about enough to fill one regular tampon throughout the afternoon. I thought it was just starting. Anyhow, my boyfriend and I had sex that night (because it was so light) and he noticed there wasn't a lot to clean up. (Sorry for too much information). This morning (the dayvi will be taking the fourth placebo pill) there is hardly any discharge at all. My question is that is it possible the sex could have interfered with my discharging? I feel like because it took two days of placebo pills (and on schedule at that) for me to get any kind of period, that it is unlikely I am pregnant. I had asolutely no bleeding or spotting before my scheduled 3rd placebo so i was not suspicious of pregnancy. Is it plausible that i could be pregnant? Why is my period stopping and starting? Help, im freaked out. =(