Why is my mare's hair standing up on her body?

I have a 12 year old quarter horse mare, and her hair all along her body is sticking up. Usually she does this because she's cold, but today it was 40 degrees. I've also heard horses do this when they're not feeling well, but she seems to be fine otherwise (energetic, eating well, etc.). She has her winter coat right now and is shedding, so could that be it? The only other thing I could think of is her front feet hurt right now due to improper shape (they're being corrected), so maybe it's because she's a little sore. Is that possible? Thanks!!!