You need to make a visit to an oral pathologist for a proper clinical examination. Any rational treatment can be instituted only after the correct diagnosis. A clinical examination is a must for the right diagnosis and might require a biopsy also. The dentist would also take complete history of your general Physical health as well as any medications or habits.
Next Steps
search for an oral pathologist and contact him soon.
Health Tips
do not delay this treatment because multiple chronic ulcers can have serious complications also.