What Causes Severe Pain After Taking Vicoprofen?

I take a vicoprofen twice daily. When I first take it, I hurt much worse for about half an hour, then the pain subsides. Why this response? After pain lets up, I feel relaxed and can lie down and sleep for perhaps two hours. After that, I have some mild pain, but not too bad, so I can get some work done. I am 86 yrs old, have had 4 back surgeries for stenosis, etc. I had my back broken when I was about 4 yrs old. Had little treatment then, as we were very poor. I did hard physical work, had 6 pregnancies and 4 live births. In my mid-60 s I turned over in bed one night and was paralysized, in terrible pain. That began the surgeries. I can function with the 2 pills per day. But WHY the pain gets so much worse, before it gets better?