I've never had this problem before. I got a new mare who was a little underweight when I got her. I put her on the same feed that everyone else gets, and she started gaining weight. Then I started working her. Light work 3x a week. Then she started looking a little too plump. So I cut back her grain. Started working her a little harder. She's still gaining. Cut her grain out entirely. Started working her heavily 5x a week. She is still getting bigger. She has a dry lot and is getting low sugar 2nd cutting hay. What should I try next?