It could just be a hay belly. But most babies go through a stage of looking like they have a big belly. You could always consult a vet to be sure nothing is wrong.
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!
I would usually call my vet and ask for his advice. My horse has quite a few digestive issues (since birth) and any time I have any worries or questions I just give him a quick call.
She may have a hay belly, most horses who eat only hay get those.
Our colt went through that . I read that it's just a phase before he has a growth spurt
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