My period just ended five days ago. the day after i was sexually active and today i got my period again. what could be the reason i'm getting it twice?

Changes in bleeding: This can occur for many reasons from hormonal imbalances, stress, infection, pregnancy, bleeding disorders, medical disorders, growths..... If it becomes a pattern over 3 months, then follow up with gynecologist. If associated with heavy bleeding more than a maxipad full an hour, fever, odor, chills sweats, shortness of breath, pain then follow up ASAP or ER