The roof of my mouth and my throat are sore and I have a blister on my gums towards the back. What could it be? I'm not sexually active. Worried.

Likely viral . Many viral infections may do this including hsv, which is common. These are usually self-limiting. Some times we find no cause and they still go away on their own. Reaction to certain medications or new foods should be considered. Rarely can this be a herald sign for a more serious underlying or for a systemic infection. See your doctor for a close up exam.
See a dentist. If these symptoms do not go away by themselves in a few days, you should be examined by a dentist--you could find there are ways to give you both reassurance and relief!