I have a huge purple bruise on my leg with no pain, but a very small ball I can feel at touch. I sprained my ankle on the same leg yesterday. What is it?

Bleeding from sprain. A sprain involves tearing, to some extent, the ligaments and possibly tendons around the ankle. Bleeding always occurs internally, but may be visible or not. The bruise with ball could represent a separate hematoma occuring with your injury, or seroma. If it doesn't go away, or your ankle cannot bear weight, see a doctor.
Have it looked at. A sprain injury can result in redness, bruising, swelling, pain, and possibly instability of the involved area. An x-ray will be able to assist in differentiating a sprain form a fracture. Have it evaluated by a professional and get the appropriate treatment.