After having sex i have lower pain in stomach and lover back almost like period cramps and it hurts for a few days after what could it be ?

Pelvic floor spasms: You may be experiencing spasms of the pelvic floor, themuscles that hold everything in place. Sometimes they get weak and fatgued and start getting really tight, like a charlie horse. Causing a crampy pain in the lower abdomen and low back. If this persists you must be evaluated to ascertain that there is not a pelvic organ problem.