My left knee was injured in a car accident in march this year, mri is clear but it hurts when i lie down why is this?

See orthopoedist: Knee pain can come from several causes. If MRI is clear, that only means there is no internal damage or fracture. You could have jammed the knee cap into its patella femoral groove, or strained the muscle insertions behind the knee, either of which could cause pain on straightening the knee, or stretched the capsule to cause a baker's cyst behind the knee which gets compressed. Good eval is needed