If blisters on the roof of the mouth are appearing, a cause should be determined. Enjoin with a medical professional to get to the root of the problem.
This is normally referred to as Mouth Ulcers. The causes can range from simple ginigivitis to mouth cancer (don't get scared, that's a slim chance). Other causes could be a case of the measles or...
The question is "how". I've gotten lumps in the back of my throat before and the doctors called it "cobblestoneing" it comes from irritation and some people are just more prone than others.
This can sometimes be caused from eating foods that haven't yet cooled to a safe temperature, but can also be due to fever blisters that form inside the mouth, rather than on the mouth.