There are many different possibilities, so it is important to be evaluated by a doctor
. Of course the most concerning is cancer, as any bump that is present for any amount of time has to at least have some consideration given to that possibility. The fact that it is not painful and has been stable for several years makes this less likely, but not impossible. That is one of the primary reasons that you should discuss this with your doctor. Other possible explanations include fatty tumors under the skin, which are sometimes called lipomas. These will generally be quite squishy, which you do not seem to be describing given that you call it a knot. Other conditions of the skin can result in tissue cysts that can grow under the skin and cause a knot similar to what you are describing, such as an inclusion cyst. These can sometimes become red and angry, however, which you do not describe. Your doctor will be able to examine this spot and provide some further assistance. In some cases, removing these is appropriate, both for treatment and also to provide an explanation about what the mass is. Please speak with your doctor to determine what the best next step will be for you.