Cancer or a cyst?

Okay anyone who has ever had a cyst or cancer let me know what you think. I’ve had this bump on my lower back and I noticed it after I had my daughter. I didn’t know if it was something to do with postpartum? Or if it could be a cyst because I never had one or if it could possibly cancerous? It’s like the size of a marble and it’s pretty hard. It hurts a lot to push down on it too. I thought maybe it would go away with time but my daughter is almost 2 and my son is about to be 1 and this bump still hasn’t gone away?? I’m not sure if I should go to a dermatologist or someone else? My parents never really taught me these things growing up. We were always fine and just walked it off. I don’t remember a single time I ever went to the hospital or the doctors unless I was getting shots or a physical 😅 now I’m really stuck not knowing if I should be concerned and get it checked out or if I should just leave it.