My filly had that when I got her. I wormed her once a month or every two months, still didn't go away. Then I asked the lady who owned the boarding place and she said most young horses get it. It's a phase. She'll grow out of it!
She may have digestive or hind gut issues. It may be good for her to go on fast track or intensive care gi. Ask your vet first though for their opinion.
It could just be that she is overeating/fed too much