barring an allergy to dairy, cocoa solids, or cacao butter, that's definitely a likely culprit

it's probably not hard to find chocolate syrup without carageenan (Hershey's doesn't have it for example) but I'm not surprised commercial premixed chocolate milk has it as a stabilizer, since chocolate syrup has this annoying habit of settling out at the bottom of milk

You definitely shouldn't be drinking that if your throat is swelling, carry Benadryl in your wallet in case you do. You should probably get an allergy test too...