Vitamin D aids in the absorbtion of calcium; with the sun coming back, she should start getting a bit thicker of shells, but if you want to help her out you can give her 1,000iu of Vitamin D (maybe use your own D supplement) mixed with oatmeal or some other treat every other morning. When my flock's eggs got thin, I started giving them oatmeal 2-3 times a week, each time (for my flock of 12) I put 15,000iu of D in it. Their shells immediately thickened up. I haven't had to do that in a long time since the sun is coming back.
Thanks so much, I will have to look into doing this. I don't have any Vitamin D supplements in my possession right now, but it might be worth getting some.