Hi! I would not assume it is an MS issue. I think it is time for you to check in with a dentist or hygienist if they are lasting more than a day or so. Some people get blisters on their gums from foods that are too acidic or an allergic response.

a friend of mine has sores all over her mouth that were painful ended up being thrush

I'm having this problem too. Sometimes my tongue hurts so bad I can't eat anything. The dentist said they are canker sores and that they are a response to the severe dry mouth I have. She prescribed a mouthwash specifically for the canker sores with a mixture if ingredients that is helpful for the pain.
But it doesn't deal with the dry mouth at all.
If I get stressed my mouth gets so dry I can't speak or swallow. No one seems to be able to tell me why this is happening or offer a solution for it. Does anyone here have this problem too? And do you have any solutions?

I have always been susceptible to canker sores, but when I took Gylenia my mouth was full of them. Thus the reason I stopped the med.

I looked up the mouthwash they gave me and it's the "magic mouthwash" that is used to treat thrush. I got a lecture about getting rest, too. :)