Can the blood clot behind my knee dislodge if I hit my shin against shower rail? Slight bruise sz of quarter with with a slight bump. On xarelto 20 mg

Very unlikely. The injury described would be quite unlikely to cause the clot you have to dislodge. The swelling at the site is due to local bleeding from the injury and being on a blood thinner. Elevate the leg, cool compresses to the injury and watch for increased swelling in your calf and lower leg.