Yes you should get seen by a Gyno and there is nothing to hide from your mother. It would be good to have her support as bad cramp pain with periods is not normal it could be a sign of Endometriosis.
Yes, tell your mom. And have her go with you to see a gyn who is female. Actually have her in the treatment room with you. When you tell your mom, just say it like you did here, ok? And ask her what a gyno doctor does, so you will be prepared for the exam. Good luck -Blu
Just tell her that you think it's time for an ob-gyn visit, and if she asks you why, tell her that your periods are irregular and you're having bad cramps.