About 2 months when closing my car door I hit the back of my left leg just below the calf muscle. The area was black & blue but now the area has a lump about the size of a tennis ball - when I press gently on it I can feel it. You cannot see it but you can feel it when I touch it and it is tender when touched.
Not to worry much as this is benign condition like a hematoma or blood-filled sac or a bursa. If really worries you can go for an FNAC test for confirmation all the best!