Why is there a bubble in my gums by my back teeth?

Probable abscess: A bubble -like a pimple - on the gums is typically an abscess (infection). There are 2 types- gum abscess (periodontal disease) and tooth abscess (root canal). You need to see a dentist to examine and take radiographs(x-rays) to determine the problem and the appropriate treatment.

Possible infection: This could be a sign of infection. Have you dentist check this area.