What Causes Recurring Tonsillitis Inspite Of Being On Penicillin?

Hi,I had tonsillitis just under two weeks ago and was prescribed penecillin. It started to work after about two days and the sore throat disappeared. i continued (and am still curently taking) the course of antibiotics as prescribed, but somehow today I have woken up with the tonsilitis flaring up again.
Any help?