Has anyone ever had a firm fixed poorly defined chest wall mass that wasn’t cancer?

I’m a 30 y/o F. I’ve had it for years.. I don’t even know how long it’s been since I noticed it. 3 or 5 years? Maybe more? It’s nontender and nonmobile. I can’t even tell how big it is or where it starts/ends. It doesn’t feel like a lipoma or a cyst. It feels like cartilage or extremely dense fibrous tissue. No overlying skin changes. I can see it under the skin when I look in the mirror and raise my arms above my head.

I had a breast reduction ~8 years ago and remember the surgeon telling me I had very dense breasts. He said he noticed a mass on the tissue he removed but pathology came back normal. I don’t remember what it said exactly.

Basically I’m looking for reassurance that it could be nothing. The GYN I had look at it didn’t feel anything but I am getting a diagnostic US soon. Maybe I’m just lumpy?