I'll be the first to say, stop using it and tell your friends to throw it away. I use cap strawberry and so do my buds. Haven't heard of any blistering problems from anyone. I honestly believe that Cap strawberry is safe to vape, though it doesn't mean a vendor is incapable of dropping the ball. Hope you find out what is causing your unfortunate experience and let us know what happened.
I can eat peanuts with abandon, but I know others that are allergic to them.
Disinfecting cleaning wipes often have citric acid in them, and they will cause my hands to get itchy and blister, so I think this is totally possible. Or it could be another compound in their strawberry flavoring. Strawberry flavor generally contains over 50 compounds.
Another possibility is if you are vaping very high nic at higher wattage, you can start to get mouth sores from it.
Or it could be a reaction to PG that has taken a while to manifest. But if this started right away with this flavoring, it is probably the flavoring. Ideally, you should try to vape unflavored for a while and see if it goes away. If not, it might be too high nic, or it could also be a delayed reaction to PG. But the flavor sounds like the most likely culprit.
In the meantime, gargling with a baking soda aqueous solution will take some of the pain away. I do this to ease the discomfort of cancer sores when I get them...actually cinnamon does this to me.