Lump on left side/rib

[AngryRobot]

I was showering yesterday and noticed this palm-sized lump on my left side rib, about the same height as my nipple. See this picture for an idea: https://i.gyazo.com/8db96d36fa244924ec0f4362d1435ff6.jpg
It's not perfectly round. Again, it's kinda the size and shape of the circle in that picture. It's softish, kinda like muscle. When my arms are down I don't feel it. It's not on the other side of my body.
No other symptoms.

I'm kinda freaked out about this. Obviously I keep thinking cancer. I'm seeing the doctor on Wednesday but still :(

[Liddy1993]

If it’s soft it’s nothing to worry about and your doctor will reassure you of that! From what I’ve read (googled) the bad ones are rock hard and stuck in place and you wouldn’t have just noticed it out of the blue I reckon it’s muscle if it’s as big as you say but I know how hard it is to tell yourself that when anxiety is telling you otherwise.