1) It is about a animal:
Gata = Female cat
Gatinha = Little female cat
Exemple: A gatinha fez xixi no carpete/ The little cat pees on carpet.
2) You use to refer to a very beautiful girl.
Exemple: Nossa! Você é muito gatinha!/ Wow! You are so beautiful!
Whe somenon calls you "gatinha" it's because that person is saying you are beautiful.