Yellow looking blisters in back of throat, i suffer from post nasal drip that won't go away, and tonsil stones. what are the blisters?

Pharyngeal vesicles: There is little way to tell what these are without seeing them. Coxsackie virus infection can produce some blister-like lesions (herpangina), but the best way to diagnose this would be to show it to a doctor. Good luck

Lymphoid tissue: The bumps in the back of the throat if they are not painful are probably lymphoid tissue which are a response to the drainage from your post nasal drip. They are reacting to viruses or bacteria.