Justifiably worried?

Hello all. I dont really know where to post. I haven't been given an official diagnosis, just a "we think this is what it is".

I'm 28 with no family history, no relatable risk factors, and 1 child. 6 weeks ago I found what I thought was a lump, I'm pretty immature when it comes to medical stuff. I've never been one to check my body or freak out about even serious stuff. But my father in law just got diagnosed with terminal cancer, 6-12 months to live. I think it freaked me out and I started checking everything.

Even now looking back, I can't believe I freaked out over this lump, it seriously just feels like a chunky area of fat. I had a mammogram (>75%) and ultrasound that came back clear. They saw nothing at all. I had a hard time processing why I feel stuff and they see nothing, but they say it's all normal tissue, lumpy but normal. No cysts, fibros, anything.

So I'm over the original "lump", but about 2 inches down from it, there's this weird flat area that I'm having a hard time describing. I can't find anywhere that describes what "thickening" feels like.

This area is pretty deep in my breast, flat like a quarter, rubbery, moveable, and has well defined and rounded off edges, probably 2cm×2cm and is no thicker than a nickel. Almost feels like if someone was to take a green grape, remove all of its insides and just lay the skin flat together.

Does anyone else have this? Could it be a deflated cyst or is there such a thing as a flat cyst? If it's from an old cyst, would it show on ultrasound?