I had the water blister like-bumps after radiation. They would come and go, and eventually appear less frequently. It's been 8 years since I had radiation, so I don't recall what my doctor said they were. Definitely check with your doctor.

Your mouth is still healing, so it is a good idea to rinse after eating - just even a salt water rinse. Also, watch what you eat. Stick with soft food, avoid spicy food, or anything that could irritate your mouth. A few months after RT, I bit into a sandwich that had a hot pepper in it and almost immediately I got blisters in my mouth. Even today I watch what I eat as my mouth is still very sensitive.