Right before my period one of my ovaries hurts?

Okay, so lately a couple days before my period starts, one of my ovaries will start hurting. Not a lot, just a very mild pain. I barely even notice it. First, it was on my right side, and I thought it was my appendix. But then I got my period and the pain went away. Then a couple days before my next period started, I felt the same pain, only this time on the left side. What is this? I'm simply curious, and a little bored. Thanks. :)