If you go to the vet, don't let them give him a steroid shot! That's the first thing they want to do for reactions!! It doesn't help & damages the liver! Rule of thumb for Benedryl if you're using adult pills is 1-2 mg per pound every 12 hours. If he is lethargic, it is the reaction & the Benedryl. Benedryl makes them sleepy & an allergic reaction takes everything out of you.
The reaction could have been caused by something you did--hairspray, candles, new deoderant, cleaning, new laundry detergent. You can take a warm damp wash cloth or unscented baby wipes & wipe his whole body down to remove any thing on him that may have caused the attack. It also helps to cool the skin from being red & irritated. Please keep us updated!! NO, you did not over-react. You're a good momma & always go with your gut. Allergy reaction are serious & can be fatal!!