Painful how? Like cramps?
If your doctor isn't concerned about it and you were thorough with your description of symptoms, then it's probably nothing to be too concerned about. Periods are always funny, even if you're "regular as clockwork" as you say. You're still young!
I've also had this happen to me when I began letting my ex "go inside of me." Which I assume you've recently started doing and weren't doing before? Maybe that's a factor.