Sometimes evolution does that. I mean, they do get bigger and stronger. Heck, it's possible she wasn't as aggressive before because she was too small to do any real damage.

[He sighs, pinching the bridge of his nose.]

But I've definitely gone through it before. Got two aggressive dragons, for one, but they cooperate... sometimes. And, actually, my Mamoswine's got similar issues. Always angry, picking fights...

[It's a little eerie, actually. This'd be the third Mamo he's known to have anger management problems.]

Have you tried letting her blow off some steam? I don't know how many badges you've got, but if you redirected her at some other target, maybe it'd cool her off.

[Then again, that doesn't always work... Depends entirely on the Pokémon.]