What Causes Intense Pain In Foot Post Surgery Despite Taking Norco?

I had surgery today on my foot. The doctor prescribed Norco for pain relief. The problem is it works ok while I am awake but I wake up in intense pain after being asleep for 60 to 90 minutes. Once I am awake the intense pain goes away after a few minutes. I am still experiencing minor pain but it is bearable. I am on a generic equivalent of norco 5-325 tablet. As instructed I am keeping my foot elevated and using ice every hour while awake.