What you have is a blood clot. Don't freek yet... They are directly a result of a hematoma aka bruise. What you can do, is use warm compresses to reduce it's size, and avoid taking asprin. If you do ride, be sure to ride easy, you can cause it to be worse. And, it can actually break loose from it's site of origin... most likely it is muscle tissue that is congested with blood and it will take time for it to reduce in size....

Good luck!