Is this normal

Im posting for my friend because she doesn’t have reddit, but this is trixi! My friend noticed hair loss on trixi’s hind legs when she got home from vacation on new years, but it seems to have gotten worse. She has also been licking and biting at her leg where the hair loss is. She appears to be thinner and has her guard up. We were just wondering if anyone has seen this or has any advice/suggestions. Thanks!