Anal sex help question?

ok so me and my bf always used to do it and yeah the 1st time it hurted but we tryed it again and got it to feel good rather then hurt, but we stoped doing anal for a wile and now when we try to do it, it just hurts and stings, like the 1st time we tryed it. We tryed it again but again with same result as pain as the 1st try. can you tell me how we can fix this problem? me and him would like to try it again some time(and yes we use lubrication )