Hello and thank you for your question. I would not think cancer is the first thought of what a lump on your sacrum could be. Different things can cause this and is it new? Does it hurt? Disc herniation can cause this type of lump, arthritis in the area called spondyloathropathy , Vertebral osteomyelitis which is inflammation or infection of the bone or bone marrow of the spine, etc. Things that you may have always had like scoliosis or hyperlordosis (sway back) can cause it. https://www.rightdiagnosis.com/sym/sacrum_lump.htm Do you have a bit of health anxiety? This is extremely common. When we constantly are checking our body or taking symptoms or different things we notice and picturing cancer or the bleakest diagnosis from them. It's treatable with the help of a psychiatrist or psychologist and sometimes medication for anxiety. But certainly talk therapy helps with it substantially. Let us know how your GP appointment goes.
Hello. I wanted to check back in and see what your GP had to say. Please give us an update.
Consider getting MRI or CT sacrum/coccyx.
If you're concerned that it may be cancer then your decision to visit GP is right. I hope the results will be good and you'll not have any cancer. So get tested ASAP and be positive!
Good luck with your tests, Best Wishes!