Cast iron skillet is flaking???

This is an old skillet but wellt taken care of & my favorite cast iron skillet.I've basically only cooked cornbread in it or sear steaks. Last week, for the first time, I used it on my outside burner to do some searing of steaks, cleaned it as usual, dried it in the oven & I noticed when I took it out last night, the surface is flaking? What causes that& how can I "fix" it? I've been cooking with cast iron for a long time & I've never had this happen.