You should probably visit an obgyn and get checked out. You could have a cyst, or it might be your fallopian tube and not your ovary that's hurting. You'd probably have to have a transvaginal ultrasound for them to get a good look at your ovaries etc. They'll probably do a blood test, and I forget exactly what it's called but I think it's ca-125 (checks for cancer).

This happens to me. Started about 3 years ago (I was 17, at the time)... got so bad that I was in tears walking up the stairs... mum took me to hospital and they kept me in for a week. I had blood tests everyday, x rays and an ultrasound, however, everything came back normal and the pain gradually went away.
I was discharged and told to go to my GP if the pain returned and/or got worse.
It comes back every now and then, doesn't appear to have any kind of pattern and no matter how many times I go to my GP, she doesn't take me seriously. She keeps putting me on the pill so she can find out 'when in my cycle it happens'... evidently "it's totally erractic, some months its happens, some months it doesn't" is far too difficult to understand!!
If you get news back on what this is, please let me know!

LAH2008--did you ever get an answer as to what caused your pain? I have the EXACT same thing! It first started few years ago and seemed very eratic. OB/GYN said it was either a nerve ending or could be endometriosis but didn't feel it was necessary to explore further. ??? Did not feel it AT all when pregnant with my 3rd child...or for almost a year afterwards. It started again about 6 months ago...seemed completely related to my period...would only feel it during that week. However, the past few months I have it at least 4 days a week...worse at some times of the day than others. There doesn't seem to be a lot of rhyme or reason to it now. Mine is a burning (almost "indian burn") sensation--seems to be about the size of a nickel or so. Have you had any answers as of late?