Lump and ache pain behind knee. bakers cyst or tumor or varicose vein?

Depends: Least likely cause is the bakers cyst as these are usually asymptomatic. The only way that a varicose vein could be felt as a lump is if it has clotted off in which case it usually would also be very painful. Therefore this could be a thrombosed varicose vein. Possibility of tumor can not be discounted as it is included in the differential.

Knee problems: The vast majority of aches and pains in the back of the knee is from the knee itself. Most commonly, it reflects tendinitis of the hamstring muscles, which is extremely common with quad muscle weakness in the setting of a knee injury. The other common source is meniscus injury or cartilage injury. A baker's cyst is a reflection of such knee problems and is often the source of fullness or a lump.