It may not just be your medication. You are right in the middle of a major energy drain!! This surgery takes a great, great deal out of the body, and major healing is going on in that knee. Bone, soft tissue, muscles and tendons. No wonder we are tired!!
I'm three months plus out and still get tired easily. Healing is going on for at least the next 9 months!
Don't try to cut your meds just yet dear one. Your body is just expending a great deal of energy healing all that has been done to it. Your meds are necessary. Try to go with what your body is telling you, and relax and nap when you are tired. It really is the best thing.