I experienced somewhat painful cramps later that day started my four day period days, usually 7. i'm sexually active. what could be wrong with me?

Pregnant?: always do pregnancy test to make sure you are not pregnant. see your obgyn for evaluation by exam and u/s as needed if not pregnant. if you are not pregnant, you can take motrin 800 mg every 8 hours for bad cramps

Probably normal: It is not uncommon for young women to have changes in the timing, duration or symptoms associated with menstrual cycles. If your cycles remain irregular or cramps persist/worsen, then seek medical attention for further evaluation and treatment options.