My border collie has a red bottom!?

About 6 months ago I noticed that my 8 year old border collie his bottom () looked very red and angry. I took him to the vet and he really didn't know what was causing it the sacs were fine no problem there, no change in his diet and his stools normal. He ended up giving him a shot of anti biotic and a steroid injection. Well I suppose it calmed down abit and I thought well i'll see how it goes. Today when we were on a walk I noticed it had flared up again. Has anyone had this problem with their dog and what do you think it could be? It doesn't seem to bother him and he is healthy in all other ways. He is on a diet of Bakers with boiled chicken and sometimes mince. Thanks in advance.