Hmmm and Eek!
Welcome
to the forum, btw, I've been on holiday so am catching up with a coffee and extra cat cuddles today.
Might be worth a quick trip to the vet. If she's not preggers this weight gain may be something you need to know about. Can you measure her waistline with a measuring tape to monitor her - it's quite tricky to gauge by eye.
Also, and forgive my scepticism about the origins of these lovely furbs, but I understand from your other threads that they came to you as already neutered from a friend of a friend.
I've heard this tale before, where a dodgy owner has told fibs about their status in order to get cats re-homed. If she is pregnant, then almost certainly the boys will be entire as well. This would possibly explain two things you mentioned:
1. The smell you experienced when they were in your spare room could well be that Tom-stink that would only be present if one of the boys wasn't successfully castrated.
2. The negative reactions from your two resident cats would be exaggerated if any of the new trio were entire.
I do hope I'm jumping to conclusions, but if there is the chance she's about to produce a set of kittens you need to know for sure what you've already (heroically I must add) taken on-board.
