Pain in lower abdomen?

Okay so I am 17 and just a few months ago I got this slight pain in my lower left abdomen. At first I though maybe it was my appendix but it is on the wrong side. I noticed it only comes around during my period, which may be a coincidence but I haven't noticed it at any other time. So I am assuming it could have something to do with my left ovary maybe? It doesn't really hurt, and I wouldn't describe it as stabbing, just a dull pain. And just now I realized that if I push the area it feels hard, while the other side does not. It hurts a little when pushed on, and the hard area is directly in line with the top of my hipbone. Also, I started my period yesterday which I when I noticed this again. Any thoughts? I know it is a problem and will go to a doctor, but for know I want to know if anyone knows what this is. Thanks for your help!