Lastnight an today ive experienced lower stomach cramps they are really bad I had sex 1 day before an during my period I finished last week what is it?

Stomach cramps. Could be a UTI ( do you have burning when urinating?) or a vaginitis. or just stomach cramping from spoiled/ food( diarrhea?) Follow up with doc urgent care today.
It is: time to get examined. Your symptoms cannot be diagnosed or treated without a pelvic and abdominal exam.