Well, now you know a little secret about recovering from joint replacement surgery. It is a BIG deal. When the surgeons talk about 6-8 weeks "recovery" they mean the incision is healed, the implant is knitting into the bone, the muscles, ligaments and nerves have begun to heal.
What they don't talk about is how unhappy all those body parts are at being manhandled, that muscles and tendons get weak and stiff, there is a lot of bruising inside and out, and that nerves take a very long time to heal. So ice, ice, ice, move the knee as much as you can, elevate I when you are sitting or lying down, and control the pain with tylenol or ibuprofen.
After shoulder surgery in October, I iced for 4 months, alternated ibuprofen and tylenol for a couple months (I'm still using one or the other a couple times every day) and I'm just getting ready to start the final phase of my PT.

Be kind to yourself! And keep moving.