I'd put my money on a few different things, like worms, or malnutrition (or not adequate to her current needs from foaling).
I'd look into the basic things before getting too upset. Is she on a good worming schedule? What is her diet like? Hair problems are usually from something going on the inside, not enough nutrients getting stored away and things of the such. It could be from worms taking all her nutrients, or it could just be she is dump and tired from that foal!! If you had a needy little thing eating off of you and bugging you constantly, you'd be worn and tired and skinny too! LOL.
Edit: If you are truly worried, asking a vet to come out wouldn't hurt. But it might just be a waste of money if it's a simple fix like stated above. If nothing else, you can call and ask them their opinion and what you should do. Good luck. :)