Louise,
I'd have to agree with hopegirl, the pain is always changing. I have my areas that are always sore, but it can migrate from there, get worse, come roaring back. I usually know right away when I've done something "bad," 
like lifting a few ounces too many or driving over a dip in the road too fast: all my "fault lines," all on the left side of my body, start throbbing and it feels just like there's a seam running down that whole side, from my toes to the top of my head, that's pulled too tight. Then I "kick" myself!! On some of these nights, it grows into the kind of pain I can't ignore. 
At this point, random muscles in my back, butt, and neck like to join in for whatever reason. (This can also happen out of the blue.) That's when I turn to Darvocet. I very rarely have to take it before evening. Sometimes I'll take 6 a week, sometimes none, but I probably average 3 or 4.
I think lethargy can be a product of fighting pain. I have more energy, move more, and get more accomplished when I DO take the meds than when I wait too long and resist it. But that's just me. I hope you figure out what's going on and what works for you soon, because it's so hard when your imagination is being held captive by pain!