Because it takes a lot of O2 to deal with pain. I don't know why it does that, but I do know that when I was dealing with the pain of untreated cellulitis (I didn't know what I had!) my O2 usage went up. When I finally got it diagnosed and treated, I mentioned that to my doc, and he confirmed that dealing with pain often requires more O2 than normal, and those of us with compromised lungs can really tell the difference between pain and no pain. So take your meds!