What Causes A Soft Lump At The Back Of Head?

I have a small lump on the back left side of my head, just above the hairline. It does not hurt and is soft to the touch. I noticed it last year. I used to wear the do rags a lot, and felt a lot of tension there. Stopped wearing the do rags now this lump developed. What I can say is I tend to lay on my left side more. My stress is felt in my neck and back of my head. Please help me.