Chest congestion in infant but not sick?

My little boy who is now 11 months old has been having off and on chest congestion since he was about 5 months not sick no runny nose or nasal congestion just sometimes randomly he will get real congested whether is last all day or something only a hour or a few hours then goes away it just comes and goes and we have taken him to the doctor who said it was prob, a sinus infection gave him anitobitocs but didnt help b.c to me its not a sinus infection if it comes and goes sometimes can be clear for a week then he can get upset crying and get congested or can excited and get congested, so im just stumped on what it can be or if anyone else's child does this and what it could be?