Hve recurring painless pimple -like inside my mouth. It occurs nly 1 at a time. Its small only and white. What can it be?

Infection. It is a recurring pimple of pus that is draining into your mouth. See a dentist and get it treated.
Mucocele possible. Possible cause of problem is a mucocele or a mucous cyst. Indirectly caused by possible trauma or blocked minor salivary gland. Area should be checked by medical personal.