Sick 5 mo old please help

My DS is 5 mos old, no hx of asthma, wheezing or just being sick. He woke me up this am in severe resp distress. Wheezing, struggling to breathe, all kinds of wierd noises. His chest was expanding assymetrically (less on left side with some retraction). All this got much worse when I laid him flat. His nose was running and drool going everywhere. His cough had a second sound to it that I don't really know how to describe. i held him straight up and repeatedly patted his back trying to loosen secretions. I took him to the ER, he was fine by the time we got there, sat 100%, no wheezing, just an occasional strange sounding cough (kinda a real high pitched noise with it). He's fine today, just feels bad and seems to be suffering from allergies with red itchy eyes and runny nose. He ended up not actually being seen at the ER because I took him back home when the RN didn't hear anything and his sat was so good. Could this have been a mucus plug? Do I need to have him seen by his PCP? He's not wheezed anymore and no cough either. TIA!

ETA: He was perfectly fine when I put him to bed, and not exposed to any new chemicals, detergents, etc that I know of.