stung by a bee, maybe? Juno loves bees so he's been stung a couple times around the nose and muzzle....luckily I saw it happen because he, too, didn't yelp or make any type of noise and his muzzle & nose just swelled up after & he kept licking it. Benadryl should help IF it's a bee sting. Keep an eye on him though as you already are. :)
Sounds like Sterling got bit by a bee, or wasp. The bendryal should help. Funny, same thing happened to Abbey yesterday. We had her outside in her play pen, and when we brought her in hubby noticed her snout was swollen. He went out to the play area and checked to see if he could find out what it was. I stayed in and checked for bite marks. Found nothing, and today she back to her usual self. I hope that nothing serious is going on. If you feel uncomfortable about it, might be best if you take him to be seen.
Hope Sterling is feeling better. No advice from me, you've already been given the advice I would give.