You could be dealing with a resistant strain. Did the cultures and sensitivity come back?
Another possibility is Epstein barr, or mononucleosis.
Hello~I would complete the entire prescription, if your throat is not better, I would see an ENT. Also, if possible, try gargling with salt water and vinegar during the day, this has been known to stop sore throats, also, take Propolis; this is a natural supplement made by bees, it actually acts as an antibiotic. Adding extra Vitamin C and Vitamin D will also help.
I hope you feel better soon.