My 4 yr old had a blister like sore on her gums beneath the teeth on the bottom and had been congested and stuff lately.. what could this be?

Sore: S he may have herpangina which is caused by a virus. Giving cold things to eat or drink will sooth the pain

2 possibilities: There are 2 main possibilities -- one is herpangina (a horrid sounding name for the 'mouth' part of hand foot and mouth disease, and the other is a fever blister/cold sore. Both of these are caused by viruses. Best is to use acetaminophen for pain; avoid juice, tomatoes, and citrus fruit because they'll sting.

Oral blister: It could be an apthous ulcer or fever blisters Or if she had many of them, it could be part of the Hand Foot & mouth syndrome