This problem use to happen to me and my sister when we were kids. The problem is too much chlorine in the pool. Normal amounts of chlorine will not do this.
We used the shampoo in the purple bottle.......that can help blonds, grays, and white haired ..........I can't remember the name though.........something lights.........shimmer lights........I think.
That worked great........instantly.....and doesn't harm the hair at all!