Pain prior to and during period on right lower side.

I previously put this down to thinking it may be a twinge in my appendix, but Ive now noticed that I get a dull ache on my lower right side prior to my period, and the ache is increased during my period, It feels like a slight numbness, but occasionally as though if I tensed something would go horribly wrong/pop. This doesnt happen every month, occasionally there is also a very watered down version of the same feeling on the left side. But it is more otfen on the right side and at least every other month for the last 6, before that I cannot fully remember
The pain constantly comes and goes throughout the 3/4 befre my period and the 5 days during my period and then is gone until the same time next month. Without looking at a date I can tell when my period is due because of this.
I have also been off the pill since 2004 without any joy of pregnancy, which is being looked into but Im really wondering what this ache really is, as I cant find anything online to say what this could be. :$