Dear patient After knee replacement your knee and hence entire leg is put to rest and hence circulation is hampered in leg. Stagnant blood in limb due to nil muscle activity leads to clot formation in in limb. So in many hospitals there is a protocol to put patients on anticoagulant therapy after major lower limb surgery to prevent deep vein thrombosis. Now since you are on anticoagulation therapy it puts you at risk of delayed blood clot formation and hence internal bleeding. Stiff neck and headache might be due to intracranial hemorrhage. I would anticoagnddvise you to visit emergency and to go for ct scan of head. All the best.