A few hours after she got back home, she became incredibly lethargic. Would sit up to turn over, but wasn't walking anywhere, wasn't eating or drinking, and wasn't reacting at a normal level to me - she'd look at me, twitch an ear, but wouldn't shift around me or grab at my hand.
Ok, I figured. Maybe she's just worn out from the vet visit, or maybe it's like humans get after a flu shot, and she'll be back to normal in the morning.
Nope - in the morning she was still lethargic, if very slightly more alert. She actually started to walk around a little bit around noon, but we took her back into the vet anyway. (I knew she still wasn't feeling well when she didn't even resist being put into the carrier!)
Turned out she was having an allergic reaction to the vaccines, and running a fever of 103.5 (normal for cats is 102). The vet gave her an antihistamine, a steroid, and some fluids, and kept her for a few hours for observation.
On her return home, she ravenously gobbled down some food, and is now bathing furiously - in short, she's acting much more like her usual self.
Interestingly, this means that short of the every-three-years legally required rabies vaccine, it's probably best now not to vaccinate her at all. (Which in turn pretty much guarantees that she's never going to be an outdoor cat, no matter where we live.)