I have this when I ovulate. It is painful and very strong and last for days. The worst is a few hours when I am sure I must be O. I can feel the actual moment I O and then the relief its happened as the pain changes into a more warmer feeling and ache like bruising. The two cycles I have had this tha worst in the last 10month I have gotton pregnant so I am hopeing to feel it again very soon. If its around O time I would be confident this is why although if it keeps going on during the cycle maybe have an u/s. I'd say its perfectly normal and its cause your body is about to release an egg.