This is a fat contusion.
It is basically a bruise of the subcutaneous fat layer.
When it is traumatised some of the capillaries in the fat are broken and blood seeps into the interstitial spaces in the fat causing the echogenic blush.
It will resolve with time.
Sometimes (like in the knee lump you posted it does not resolve completely and you can get cystic spaces forming and the lump may persist for years and this is a fat necrosis event. You see this in subcutaneous fat and also in the female breast where it can form an “oil cyst”
Another option is a morel lavallee lesion where the deep and superficial fascia of the fat layer seperate and you get a chronic accumulation of blood and lymphatic fluid into a chronic cavity (a bit like a post traumatic seroma). These can be hard to treat as when you drain them the re-occur.
Another option in your case if there was NO trauma (and I think there most likely was trauma in your case) is fat inflammation which can be termed panniculitis. This can occur for any inflammatory reason. A bite, mastitis, appendicitis are ones we see regularly where fat is inflamed.
Steve