Update: She just got up now, it's 9:00 pm, and she seems to be recovering from whatever it is she had. Her breathing is quiet again and she is moving around, although only slowly. I'm pretty certain now that it was a cold that she caught from me. She seems to have beaten it, anyway!
. I'll be keeping an eye on her the next few days, but I really don't think it's necessary to go to the vet now.