Pain during masturbation..

Odd question.. I'm wondering if anyone else experiences pain while masurbating? I use a vibrator and I masturbate clitorally, everyday at least once before bed, sometimes twice if I have a free afternoon. My pain only happens the week before and during the week of my period. I'm fine until I orgasm and then have a slight pain in my uterus/ovaries area.. This started happening about 5 months ago and hasn't seemed to go away. I'm wondering if anyone else experiences this? Should I be worried? It only happens when my period is expected and during. I'm not pregnant, or anything like that.. Thanks!