Could be a number of things, from an injury/sore back to the sweet feed like you mentioned. When Sandie was put on sweet feed she went NUTS...she went from being a super calm horse to throwing me off her back out of nowhere one night!
So I had them back down the amount they were giving her, and after a few weeks of reduced sweet grain and working her on a regular basis, she was back to her normal, calm self again!
BTW I think you and pony look cute together! She doesn't appear 15hh in the picture, but maybe it's because you're tall and it's making her look shorter? To put things in perspective, we measured my horse (err, pony technically I guess by her height lol), Sandie, and she is only 14.1hh. Although she looks bigger in my pics because I'm only 5'1"! I just LOVE ponies!!