Sudden Change in Taste?

[Magnus]

So I've had Magnus on Iam's Puppy food (dry) for about 2 months now. And I know it's not good food but last time I tried switching over to something better, I don't think his tummy liked it and was painfully constipated. (He had rectum issues so I'm extremely paranoid about his bum/constipation)

He got his rabies shot on 2/28 and seemed to be handling it well. Ate/drank fine and Monday morning, he got scared from some yelling and what not. (Not directed at him but it was loud) After that, he won't eat his food. He'll eat his treat and pretty much anything else. (ex. apples, some veggie) Poops/pees fine too. So I don't think he's sick?

I figured he'd eat his food if he's hungry enough but he ended up not eating his food at all for about 1.25 days. I gave in last night and ended up giving him some boiled chicken/rice and he totally gobbled that up like no tomorrow.

He plays fine and has lots of energy. I can't figure out if his sudden change in taste is from the rabies shot or from the shock of seeing/hearing all that yelling. Would that even be a factor?! Anyone ever dealt with something like this before?