How To Treat Irritating Knots Under Skin?

I have these irritating knots under my skin on my rear end, they seem to flare up out of the blue not puss filled or anything they just irritate then within a few days to a week they seem to go away. Some of them have left small purple scars and it seems as though some of those have disappeared. What is this and how do I get rid of them now and prevent them in the future? I thought they were boils at first although they are not puss filled so I am not sure, my doctor says they are external hemmroids although he did not look at them either, upon looking them up online they absolutely do not look like those, so I am at a loss can you please help me? Thank You Teresa