Hurt knee, calf is swollen and sore to touch. had ultra sound but there's no blood clot. what could be wrong?

Water Dominant: Our primary vehicle (body) is majority water molecules, typically ~55-62% (greater to lower fat). Thus any new swelling most likely water accumulation, edema, in the area. Blood, water with added salts, proteins & cells, is only ~7% of total body, thus only possibly involved. Us commonly used to see clots in veins, not in tissues. Us cannot see clots in tissues, especially if diffuse, just echos.

Swollen knee/calf: You could have a ruptured baker's cyst (a common cyst that can occur in the back of the knee). Swelling moves downward pulled by gravity, so any swelling in and around your knee can cause this as fluid tracks downward. See if your doctor agrees.