Cold with a fever

Hi, My 17-year-old son has a mucousy cough and an axillary temperature of 98.4 today.  5 or 6 days ago I took him to see the doctor, and he prescribed penicillen, which my son just finished today.  The problem is that while he didn't have a fever when he went to see the doctor, he has one now.  Does that mean the antibiotic did not work?  Does he need a different antibiotic or just time to heal?  Also of interest, the doctor did a strep test for my son when we went in to see him, but we did not hear back from their office, which the doctor says means that he did not have strep.  They tested for strep because around this same time,  my son had contact with a girl who had strep throat. She is presently in the hospital for lango(?) tracheitis.