My 14 month old has had congestion and cough for about 3 weeks and then got what seemed to be significantly better ... Two days ago he had fevers and still has a lot of congestion in chest and nose. Dr started him on Zithromiacin yesterday and he has had constant unbreakable fever for 24 hours now and is very lethargic with little to no appetite. He has been burping a lot too which is weird. Should we be concerned that it may be anything besides a sinus infection as te doctor suspects?