1 enlarged tonsil filled with yellow stuff. sometimes gets painful when swallowing and can feel it in my throat had penicillin twice. why won't it go?

Tonsillitis : you may need to see an ENT specialist since it may be a sign of abscess in a tonsil or peritonsillar area, which may not respond to antibiotics unless proper drainage performed

Stone?: Perhaps the yellow stuff is a tonsil stone which is food being devoured by bacteria. Or perhaps it is mucous, or mono tonsillitis. These will not respond to Penicillin. I suggest having your doctor recheck you and examine for these possibilities. Good luck.