Why do i get little blood filled balls in my in my mouth. i started getting them since my last dentist visit , could it be an infection?

Trauma: These blisters are probably due to mild trauma of the oral mucosa during your dental visit. They are harmless and should resolve on their own. If this condition persists for more than a week or so, go back to the dentist.

Unlikely: Most likely its from biting your lip/cheek while you were numb. There are other things that could be a possibility as well such as trauma as stated in another answer, or even an allergic reaction (thinking latex gloves), though this would be relatively unlikely if the blisters are blood-filled. Either way, i doubt it's an infection.