I think i have done something to my 5th metatarsal, very sore, cannot walk properly. just had an xray & ultrasound - both clear. what could the issue?

Metatarsalgia.: Just because the x-ray and ultrasound are "clear" does not mean the metatarsal is not injured. Aa rather recent injury may not display evidence, especially in stress injuries. If it is hard to walk, then don't do it. If you must use a cam walker boot to help stabilize the area and allow the involved tissues to recover. Rethink the possibility of overuse of the foot causing a metatarsalgia problem.