please help

My period is almost 3.5 months late (I've missed on period before so irregular periods aren't new with me) but this is the latest I've ever been. I'm gaining a lot of weight especially in my lower abdomen area. My boyfriend and I have sex a lot.. but I haven't had any morning sickness and I've taken several pregnancy tests but they're all negative?? My boobs hurt like crazy especially my nipples and they're turning a tanish/brown color. What do I do?