This happens to Ellie every year as she moults, which is around Feb/March.
Each year I worry and wonder what else she needs, is it cos she's pregnant or what. And then it resolves itself, the kids are born, her coat grows back for the summer - and I remind myself that it happens this way each year

Make sure she has good food and a good mineral supp. But know that this is what happens when they moult, it's almost like they moult a layer of skin too. You don't need to do anything unless you want to (see Wytsend's shampoo idea if you do).