Sorry to hear you are having to endure a knee surgery.
Often times we feel surprisingly well on the surgery day (as a result of adrenaline and anesthesia) and try to wait for the pain before taking the prescribed medicine. When the pain then really sets in, you are behind and it is really tough to catch up.
I would take it very easy today and take the medications as prescribed (every 4 hours if necessary) and use lots of ice and elevation. Surgical pain is different from the pain of an injury and I remember it surprising my too after my first procedure. Also be prepared for blood rush pain when you do get up. You may want to move from laying down to sitting in stages to allow your leg to handle the feeling as slowly as possible. As upsetting and scary this is, it doesn't last throughout the entire recovery.
Hopefully you'll be feeling better (or have the pain better controlled at least) in a day or so.
(Disclaimer: if you have been taking all medicine as prescribed - even through the middle of the night and you still have significant break through pain you may want to alert your doctor's office with your symptoms just to be sure nothing else is going on.)