Kitten is congested after spay surgery?

My 3 1/2 month old kitten was spayed yesterday, and we brought her home today. In my opinion, she is too young and too small to be spayed, but it was part of the adoption contract at the shelter. Anyway, when we brought her in day before yesterday, she was perfectly healthy. When we brought her home today, we noticed that she sounds terribly congested, and has a cough. Could it have something to do with the surgery -- problems from the breathing tube, or whatever? She is acting fine otherwise, eating, peeing/pooping, and playing. Any ideas what's wrong with her?