Could a skin tag under my tongue be cancer?

Unknown: Difficult to answer without seeing it and knowig general and social history. I would recommend that it be evaluated and, if necessary, biopsied by your local otolaryngology (ENT) doctor.

Not likely: A "skin tag" is probably just a benign mucosal lesion and nothing to worry about. However, without seeing it, it is impossible to know for sure. Ask your dentist or doctor to take a look.

Would be rare: Would not be the first thing that i would think of. But you should raise your concerns with your friendly primary care physician.

Tag/Tongue/Cancer?: Best to be examined and have tag removed and sent to pathology.