I don't have any experience with lactating mares, but I recently took an equine nutrition course, and the instructor told us that mares in the early stages of lactation require twice the energy as a horse at maintenance (ie. not pregnant or working). After 12 weeks, the mare will require about 1.75 times the energy as a horse at maintenance.
This is also significantly higher than what she required while pregnant, so if you didn't up her ration, that's probably your problem right there. Her total feed intake might go up to 2.5% of her bodyweight.
I agree it's worth consulting a vet if you don't have experience with this (my vet clinic is great - I can call them with a question at any time).
Make sure she's getting a good quality forage. The instructor also suggested feeding a "stud mix" (this was a UK course), but make sure it doesn't comprise more than 40% of the ration. I would guess the same would go for any grain you're feeding.
You might also want to try adding oil to her ration to up the energy. However, I would definitly talk to a vet before doing this. Oil needs to be added slowly (upping it by 100 ml per week according to this course), and I believe you'd also need to add vitamin E to keep it in balance. You also shouldn't add any more than 100 ml per 100 kg of the horse's weight.