Have pain behind left knee hard knot, hurts when moving it or foot, been this way for four weeks getting worse. what could it be and what should i do?

Get it fixed: The two likely causes are a blood clot or a knee joint cyst/rupture. If a clot, you will need to get help to get it dissolved, as it presents a danger of moving to our lung and seriously injuring you - even death. If it is a ruptured knee cyst, it can easily be drained and give you some relief. Diagnosis is easy -- so don't delay,