I suggest mentioning this issue to your primary care doctor
the next time you see them, as there are several potential causes of this bump. Your doctor will be able to examine your knee closely and help you decide if further work up is needed.
First, this could be a sign of arthritis
in the knee. "Wear and tear" arthritis --also known as osteoarthritis -- begins to be a quite common problem in middle aged and older adults, and it may in part manifest as stiffness in the joint together with small bumps or deformities, which represent to the gradual damage over time to the joint.
Second, this could be a cyst associated with the tendons or bursa (fluid-filled shock absorbing structures) around the knee. These may occur again as a result of wear and tear or they may occur spontaneously.
Finally, this could be a cyst in the deeper layers of the skin that is not in any way related to the knee joint. Cysts commonly occur under the skin basically anywhere on the body, and this could be the case for you.
Your doctor can determine whether this bump needs to be biopsied or surgically removed, or whether it is ok simply to observe it for now. Please book an appointment soon.