I Had Sex In January It Was Protected, I'm Not

I had sex in January it was protected, I'm not on the pill or anything but the last 3 months I'm bleeding really really light, only bleeding when a tampon is there, nothing on the pad, I've took a pregnancy test and it came bad negative I don't have any symptoms of it. I feel like I've put weight on but nobody thinks I have, I'm Freaking out, why is my period doing this?