I Have Several Hard Lumps Under My Skin Very Low

I have several hard lumps under my skin very low on the left side of my back. They feel like they are connected to something and wont move. I noticed one a few months ago I now have a few more and one large one about the size of three marbles put together. The other two are smaller. I know I need to get into my doctor but am wondering if anyone may k ow the cause.