Wow, this just happened to me (and tatssxxx, aka Sandra).
Took my kitty, Wylie in for vacines and such, and a few days later he was sneezing.
I kind of ignored it for a couple days, but it did not go away.
Took him to the vet, and it turned out to be a Upper Respitory Infection. They can be serious if you don't get them on antibiotics soon enough.

He is an inside only cat also, and the vet said it's not uncommon for a cat to pick one up at the vets, even if they are there only briefly.