If you are have taken any form of immunosuppressant such as steroids or have had chemotherapy this last year, then I would play safe and seek medical care faster than perhaps in other circumstances.

I would be worried about possible infection, although you say that there is no noticeable change in temperature.

So if we exclude that, then the recent history of trauma would indicate that this may be a reactive hematoma or seratoma. So possibly a referred injury from the fall, or fluid slipping down the fascia of the shin from the knee may be the cause of this bump.

If you are able to bear weight normally on this leg, there is no residual pain and there is no restriction to movement when bending or stretching the knee then a visit to the doctor is less urgent. However with your history of kidney cancer, then your bones may be weaker than others and I would want this looked at. So I would ring your doctor and explain the circumstances and see whether he wants to see your before the weekend, or wait till Monday. Ideally as well as a physical examination, then an x ray or CT scan would show exactly what was happening, but these might be unnecessary once your doctor has looked at it.

I am sorry not to be more specific but this could be a number of things, ranging from a simple bruise to a possible infection or fracture.

Regards