What's up with my cat?

My cat Tao (female!) has recently put loads (and I mean loads!) of weight towards the back of its stomache- which is odd because it's normally quite active and has always been very thin (it's about 5 years old now, so not really a pensioner!). It's also started to be a lot more loving, and seems to want an awful lot more food from us (while it was always the passive one and my other cat begged for food, now this one is always by the food dish and first there when we give them wet food).
My first suspicion is that it was pregnancy, especially since a boy cat has been sneaking into our house an awful lot (it's owners shut it out for the night, we have a cat flap and regularly forget to shut the outside door, it's only natural!)- but we've had Tao fixed when she was little. It's odd also because I swear one of her nipples (is there a better word for them!?) was swolen, pink and visable today when I was petting her- but upon closer inspection afterwards, I couldn't see anything amiss.
Is it possible for fixing a cat not to work properly? Should her nipples be large and visable even when I 'stand her up' to inspect her, all the time? Could it be something else? Am I just being extremely paranoid?

Welcome to Pet Talk! Perhaps a visit to the vet is in order if her appearance is so not normal and concerns you.

I think I would take your cat to the vet.... Maybe as with HUmans.... some little devil got thru and your little girl is about to be a momma. OR she is having False pregnancy. EIther way.. I would take her to the vet asap.

Mommacat.. Deb

Putting on weight, as a "Belly Flap" is normal for "Middle Aged" cats that have been spayed or neutered. No matter how much baby begs for treats, don't give in with fattening foods. Just like people they get that "Middle Aged Spread" of my 9 babies, about half are 6 years old and older, and have small "Flaps" treats and too much food will do that... Isn't it wonderful how the kitties are like little humans.... :)

It is normal for a cat to get the belly flap - when she runs or walks, does it swing back and forth too? Even my older neutered male cats have this. As for being more affectionate, the cold weather has set in and most cats become much more lovey in the colder months.

If she goes out during the day, its possible that she's been eating food put put for other peoples cats. My cats have gained a TON of wieght since I got the kittens and they've been chowing down on kitten food.