What Causes 3 Weeks Bleeding After First Intercourse?

i'm on the pregnancy pill and have been for a couple months. i just recently screwed up my system by getting very dehydrated. this gave me awful cramps after my period. a couple days later i decided to have sex for the first time (bad idea i know) and after that i was spotting, which i heard was normal. but now it's like i have my full out period and it's about 3 weeks early... what's going on?