On penicillin for tonsillitis since yesterday, still really strong pain. now have ulcers on back and sides of tongue?

Virus not strep?: The ulcers suggest you might not have strep throat, the main kind of pharyngitis or tonsillitis that responds to penicillin. Ulcers suggest a viral infection, like herpangina, coxsackie virus, and others. A new infection with oral herpes also could do it. Return to your doctor ASAP for additional exam, testing, and perhaps treatment with an anti-herpes drug like valacyclovir. Good luck!