K.H.
When you get a cold and become congested it is supposed to drain through the sinuses , but sometimes these can become blocked and cause and infection , sometimes it will be a sinus infection , ear infection or chest infection , it's just one of those things that most people have had at some point. The ped has referred you to an ENT just to make sure all ok , but chances are that it is just the first course of antibiotics didn't quite clear the infection and after this second course all will be fine.