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.