We've had a similar problem with a few of our kitties as well over the years...After getting their shots, some have had upset tummies, ran a low-grade fever, seemed lethargic, or were off their food a bit. The vet(s) said the same thing...it was probably the vaccines and to watch them...So far, all bounced back after a day or two. I suppose cats react to vaccines the way people do sometimes. I've felt icky once in awhile from a shot that I've received...sore arm, low-grade fever, headache, etc.

I hope Diego is back to his old self soon!

Judy