Alright, I've figured it all out.
Cat rambles
The unexplained jumping was not asking for food like usual, but Melbcat wanting me to fix something that wasn't immediately obvious.
She wanted to get into a tiny nook in the wardrobe that was occupied by a bag of stuff, and was asking me to move it. I had seen her interest but thought it was too small for her and didn't want to risk her springing a leak in an annoying spot... but as soon as I did she crept in there and crawled behind the open frame chest of drawers (in a space made by a pulled out drawer). I'm going to have to organise/declutter the wardrobe and put pee pads down underneath just in case. I'd put a hidey box in there if there was enough room.
She will eat plain cold cat food from the tin with nothing else added so I don't know if she's objecting to the usual warm water, the tasteless medicine she's usually fine with, the tasteless stool softener, or the pumpkin baby food she normally loves. However skipping or delaying her medication is keeping her in a cycle of feeling terrible. It's not encouraging her appetite, plus she needs to have food with her antibiotics and really needs something to move her bowels.
So I've been a horrible human and thrown the skipped meal out, syringed dissolved medication and stool softener into her mouth, and given her some of the plain cold cat food. Hopefully a few days of this will get her back to normal. She hasn't finished her course of antibiotics yet but if she continues acting funny after that and doesn't become more regular soon it's back to the vet.