Not pregnant, so what's going on?

Sorry to trouble you all, but I wonder if anyone else has had this. I am over a week late for my period but my stomach has swollen into a bump shape. Pregnancy test is negative (tested first thing in the morning with a decent test). I keep getting period like cramps that come and go in waves, but they are concentrated all on one side (the left) and seem to really make my hip ache deep inside. Sometimes I am struggling to go to the toilet (both no 1 and 2 - sorry TMI) and feel like there is a pressure there. None of this is making any sense to me. Has anyone had this? Any idea what it might be? I don't want to disturb the doctor unnecessarily. I do have PCOS but post-baby (who is now over 2 years old) my periods have been very regular so I'm not sure what's going on. I don't feel pregnant so it can't be that. Any thoughts?