I agree that at 5 months she should now be wormed, if you haven't done that already. I always start out with something gentle like Safeguard the first time, and then 4 weeks later use a regular Ivermectin wormer. During her weanling time, she should be getting some high protein. If she's still on the mare/foal feed, try adding some Alfalfa pelltets to her feed, as it will give her a protein boost, and is a tummy soother also. With a high protein diet, you should start to see her blossom in muscle tone and strong bones, and lose any "hay belly" along the way. Always with these weanlings, you want to continue "feeling" her with your hands, especially in the winter, as what seems to be a "fat" horse usually isn't during the winter, and they can become quite thin under the fluff. You're doing great, and just boosting her with some alfalfa pellets in her feed, or adding alfalfa hay will be good for her, and you should see some positive changes in her appearance.