This may be apthous ulceration
They are small, painful, oval-shaped ulcers on the inside of the mouth that appear red, white, or yellow in color
They are usually harmless and heal on their own in a couple of week..it occurs
due to Nutritional deficiency or stress..
If blisters are present over body may be Harpes Simplex
Next Steps
consult an oral surgeon or dentist for proper diagnosis if very much painful..
Use an antiseptic analgesic gel TDS..
Multivitamin syrup also helps
Health Tips
If doesn't subside after 2 weeks definitely consult a Doctor for proper medication