What Causes Delayed Periods?

me and my girlfriend had sex last wednesday without the use of a condom however she is on a oral contraceptive. the next day she went to the doctor and was diagnosed with mononucleosis and strep throat and prescribed an antibiotic to treat the strep throat. Today she was expecting to start her period and it didn t come. Is there any reason for this related to her illness which has weakened her significantly and the antibiotic or is there a likelihood for pregnancy?