I found a very strange solution to this!!! She was at it again and I recorded her howls to show the vet - then I played the recording back and my cat stopped dead in her tracks, sniffed my phone, and laid down on the bed to go to sleep.
I can't believe her reaction to hearing herself howling...
So... My spayed, 10 year old tabby cat lives in the same house she has lived her entire life in. About six months ago I switched bedrooms, and she seemed alright with the switch. No other changes have happened since then - no new people, no new animals, no new schedules on my part.
However...