Have just noticed sore patch on my golden retriever's rump. He has obviously been licking it a lot (although I hadn't noticed it until now). Can't see any obvious cause, i.e. insect bite, scratch, etc. just some hair missing and quite raw. Vet's closed at weekend unless emergency. Any ideas? (dd says neighbour told her retrievers are prone to skin problems - is this true? (he is 4 yrs old and never had anything like this before.)