She's hit sexual maturity and she can see her reflection there. Looks like she's decided its time to challenge the 'rival' she see's
When the hormones kick in some females can become as feisty as males. I have seen a couple of females in the past who actually behaved like dominant males when introduced to a potential mate and actually tried to dominate him. This tends to happen more so with females on the larger side.
The fact she can see her reflection is probably causing the stress and will also account for her eating being slightly off - your girl isn't happy theres a rival around!
Just keep your eye on her and as long as her eating doesn't stop altogether and she's not losing condition I wouldn't worry about it