Hi.. Can understand your concern.. As per your complain if you are tested negative for strep throat you may have infected with virus and the symptoms which you are having are seen in viral tonsillitis too.. Tiredness and lethargy can be caused due to two reasons.. It may be due to the infection which you have or it may be also be happening as a side effect of the medicine z pac which contains azithromycin and tiredness can be a side effect of it. I would suggest you to consult your doctor again and tell him about your immediate symptoms so that he can find out that whether infection or medicine is the cause for your tiredness.. If strep infection is found negative then you shoud get investigated for viral infection and if viral infection is found positive azithromycin intake can be stopped as it is antibacterial medicine and it does not respond in viral infections. In case of viral infection antiviral drugs are of less value and othe treatment guidelines can help in healing like Warm saline gargles, drinking plenty of water,avoid hot and spicy food, should avoid smoking and alcohol if you take. Many a times the medicinal side effects disappears once the medicine is discontinued.. Hope your query is solved.. Thanks and regards.. Dr.Honey Nandwani Arora.