Ok I had intercorse with my girlfriend 3 weeks ago. About 1 week after we had sex she began her period 2 weeks early. The first day, the color was normal red, but the second day, the color was lightly brownish. Iv heard of some things about implantation bleeding. Do you guys think I have anything to worry about? Shes already taken one pregnancy test and it came up negative. But still... She is also having back pains (she calls them kidney pains) and her head hurts a lot. I guess its been about 3 weeks today since we had sex, but we also used condoms, and we changed them out every 10-20 minutes... I never even orgasmed that night. And I was very careful about were I put the condoms afterwards...