What Causes Pain During Menstruation?

I had a rather large polyp removed from my uterus two weeks ago. I rested for three days and felt fine I had no bleeding and slight pain. Then on the fourth day I got my period (which was due) it was rather painful but nothing I wouldn't expect considering. It's been almost two weeks now and the bleeding has stopped but I have a slight pain in my side and back. It's not there all the time it seems to come and go but it's more of a stabbing pain than a period. I'm wondering if I
Have an infection?