What is my kitten's illness?

details:

- male

- 10.5 months old

- purebred maine coon

- indoor only

- all shots and everything up to date, he's very well taken care of

- schedule to be "fixed" next month

symptoms & non-symptoms:

- WAS throwing up foam, but now hasn't for about 10 hours (first vomit was actual food, then everything after was white foam)

- WAS breathing very rapidly, and even panting heavily in the car to and from the vet. breathing appears to have calmed down considerably since. i'd say it's almost normal thank goodness.

- lethargic

- no interest in food, but drank a little water

- has trouble getting comfortable, though this has calmed down a bit since last night

- ONLY lays down upright or straight on his stomach. he refuses to lay on his side, which is his usual position

- touching his tummy and throat do not appear to hurt him

- he's not hiding, he's out and about, but sits very still

- no meowing or noise (he's not a big meower anyway though)

- he gets very nervous in cars, and urinated and pooed on the way home from the vet. no blood in either. the stool was a little hard.

- nothing is swollen

so the vet this morning said he could have a ragweed allergy (?!). i'm in rural illinois and we haven't had the windows open for awhile because of the heat, then yesterday it was cooler so we opened everything up. it was around this time he started feeling bad, and ragweed has just heavily sprouted up here. i just don't understand why a pollen allergy would keep him from laying on his side.

i started to think it was constipation because of the hard-ish poo, but the vet said that when he felt his abdomen he didn't feel any "backup". the vet did give him some hairball goo, and i'd given him a little petroleum jelly earlier, but no hairballs have shown up, and he hasn't needed the litter box since the car ride this morning.

right now i'm just supposed to watch him, then take him back in if he continues to feel bad. the vomiting is over and the breathing has calmed, which leads me to believe he's getting better, but but he's still just seems miserable. i just wish i knew for sure what it is! the fact that he doesn't want to lay on his side and/or all stretched out is very alarming!

oh, and his temperature, heart, lungs, and gum color are all completely normal.

any ideas?