There are multiple reasons for oral ulceration, especially when they are more than 1. A proper clinical examination with the detailed history is the most important step for arriving at rational diagnosis. So basically a visit to the dentist for an examination is the way to diagnose what caused it. For temporary symptomatic relief you can use anaesthetic ointments like dologel CT for topical application on the ulcers. You can even post a good quality clinical photograph of the ulcer on Practo for an online consultation with a specialist. Regards.