Foamy mucus - no other symptoms and now disappeared! Treat anyway?

I was walking the goats today and noticed that our month old kid had a lot of foamy mucus coming from his nose and was coughing up thick phlegm. I took a pic and sent to the vet, who said probably pneumonia and prescribed thiamphenicol. I went to pick it up, but when I came back the kid was running around, mucus-free, playing, eating and drinking, no funny breathing... basically if I hadn't seen that short mucus episode I wouldn't have suspected a thing, he's his usual bouncy and boisterous self. My question is: should I treat him anyway? I am really loath to use antibiotics unnecessarily...is it safe to wait a day or two and see? I've never seen pneumonia in goats before so this is all new to me. Thanks in advance!