A "knot" on your knee could be a number of different things. I would recommend you follow up with your primary care doctor
at home. This should be evaluated.
The cause of the knot depends on where the knot is. If its on the front of the knee, this could be Oshgood Schlatter Disease. Oshgood Schlatter is a condition where the tibia (major bone of the lower leg) actually forms a little bump that looks like a hard golf ball. This is caused because the quadriceps inserts on this location, and young people (often med) who play lots of sports can cause repetitive stress on the bone (constant use of the muscle) which causes the bone to grow into a little bump. This is not a break, but really just a normal growth of the bone. If it causes pain, there is numerous medications, physical therapies and even surgery
that can be done. If it is not painful, often there is very little to do. This feels like hard bone ... because it is.
There are other causes. For example, if it is in the back of the knee it could be a blood clot (or deep vein thrombosis). It could be a Baker's cyst (a fluid collection).
Talk to another doctor. This should be evaluated.