I have a hard piece of skin that is white on the top of my frenulum in my mouth and it hurts what could this be?

Cancer: You should see a doctor. This may just be thickened tissue there, but mouth cancers from tobacco or other causes can present this way.

Needs to be seen: It's really impossible to know let alone to guess without actually seeing you in person. These types of situations require a one on one clinical exam. See your dentist or an oral surgeon for an evaluation. Pain would indicate to me the possibility of an underlying infection.