It turned out to be Tortie having a fit :-( For some strange reason* she likes sleeping on the pad and started to fit while using it.
We took her to the vets this morning. We explained that she'd been off colour for a day or so but nothing drastic. She was sick yesterday morning and didn't eat all day, but that's all. Nothing could be found to indicate anything specific so it was a couple of shots and back home.
We have another appointment at 9:10 tomorrow to check on her progress. To be honest, the way she's acting now, you wouldn't think she'd been ill.
* Well, she is a cat.