s eating a lot and not putting on much weight, it could be
worms, diabetes, or overactive thyroid. Since she was a stray, the odds are that she has
worms-especially, if she's thin but has a big belly. In the US, worms are very common. I noticed you are from Brazil, maybe there are other kinds of worms that kitties can get there.
My best advice to you is to do all the research you can on cats and learn as much as you can about them. Here's a good place to start-our health pages.