What else can cause random pain behind your knee besides a dvt with no bumps or redness? had 2 negative d dimers last month, knees ache

Fluid/ cyst: A common cause of posterior knee pain is fluid accumulation between the muscles in the back of the knee(popliteal fossa). Sometimes the fluid is difficult to feel and pain can be ill- defined. Sometimes cysts (popliteal or bakers') will form; usually as a consequence of an intraarticular problem ( knee meniscus tear, arthritis). See an ors for evaluation.