7 years ago, i had a mole on my arm that shrunk, then completely went away over period of months. is this normal? could it have been cancerous?

Unlikely at your age: Moles (nevi) can come and go over time. Many are congenital (present at birth). Very rarely, melanoma can undergo regression (resolves), but this is usually a bad sign, as it usually means it has metastasized to the lymph node. But melanoma is nearly always a disease of older people. Unless you had irregular size and shape, black color, enlargement, you don't need to worry.

Moles: If you can't see it anymore, it is probably not cancerous. If it recurs, see your doctor.