Questions about my finch i got 2 weeks ago?

The most likely reason that your finch has become "meaner" is that it is lonely. You should buy another finch to go along with it. If you do not want them to breed, just get two finches of the same gender. To determine the gender, you must first know what type of finch it is. The most common finch, the zebra finch, is actually very easy to tell: the male has orange/red spots on the bottom and is pretty colorful while the female is completely grey. I hope i helped:)