I Went To The Doctor's A Few Days Ago Because

I went to the doctor's a few days ago because for the past week I've been running a fever, and have had this terrible sore throat. When I got tested for tonsillitis, strept, and the flu they all came back negative. I made the decision to take the penicillin v potassium 500 mg and magic swizzle mouth wash for the pain. Since I've been taking both medications I've noticed the white discoloration went away but now they look like black ulcers. What does that mean?