Hello!
I can give you some of the reasons for a knot on the knee cap, but the final diagnosis can be made by your orthopedician after an examination and if required by following up with some scans.
It can be an infection, sebaceous cyst of the skin, Baker's cyst, arthritis, fracture, haematoma or callous formation.
I would suggest you to rule out tumors and recollect if you had any past or present injuries.
Take care!
Have you been to an orthopedist to discuss having it removed?