Having had minis for 10 years. If they lose weight while nursing I usually Worm, increase grain. 12% sweet feet and 12% pellett . or something close to that %. increase hay, I do not feed bran at all unless I find the manure harder than normal. I feel it just dos'nt allow the intestines time to absorb nutrients from grain/hay etc.
How old is this mini mare. Is she chewing her food properly? It is possible that she might have uneven teeth. even at a young age they are prone to mouth/
teeth uneveness. Another trick I use in the foaling months believe it or not is bread.
I get if from warehouses after it gets pulled from the shelves. Each mini gets 1 loaf broken up. plus their grain in AM only . they love it a puts weight on, coats shiny and dosn't cost anything. Good luck