sunrisegirl76,
Unfortunately, it could be several things. You do not say how big the lump is: pea size, grape, half a golf ball? But, anyways.
It could be a resolving hematoma, which has formed some fibrous tissue with in.
I very small lump could be a phlebolith.
Moderate size lump could be a ganglion cyst, or similar outpouching of the joint capsule and synovial fluid. You can also get ganglions off of tendon sheaths.
You could have also contused the periosteum of the bone severely, which has now healed with a bit of scar tissue (fibrous tissue).
There is not a lot of muscle tissue in that region, mostly it is just tendons, fascia, subcutaneous tissues, and blood vessels/nerves.
If it continues to bother you significantly, you might want to have an orthopedic surgeon take a look at it. You may even need some studies (x-rays, MRI) to tell exactly what is going on. Good luck.