After the first 24hrs on pen it cleared up right away, the day after the 10 day regimen the sore throat is back in full force. Any ideas?

Resistant Strain? Many bacteria that cause sore throats are showing increasing resistance to penicillin. It's possible that you had an infection with some bacteria that are resistant, the majority were killed off , but the resistant strain has now come back with a vengeance. You need to return and maybe have the doc culture your throat.