Mouth ulcer could be due to many reasons like trauma from adjacent sharp tooth, indigestion, any vitamin deficiencies etc. Need to know full history before diagnosing anything. Apply Dologel over it for two to three times for three days. If it doesn't heal within a week then consult doctor online or offline for better guidance and treatment options.

Mouth ulcers aren’t contagious and usually go away within one to two weeks. However, if you get a canker sore that is large or extremely painful, or if it lasts for a long time without healing, you should seek the advice of a doctor.

We need clinical examination

There are multiple causes for the same. But needs proper clinical examination and history about it. Maybe there's sharp edges on teeth that's traumatizing.