Happening to me right now, its most likely your ovary releasing the egg. It should switch sides though every month (this I just learned). Mine is 98% of the time on my left side, which I will be getting checked for a cyst soon. My periods are abnormally painful, and my ovary hurts going into my period which I also heard is a bad sign. If you don't feel comfortable see a doctor, if it's just aching I wouldn't be worried, you'll start your period within 1-2 weeks.