I'm gonna go way out on a limb here. But, first off, pain killers do absolutely nothing to the source of the pain, they just block you from feeling it (which is an absolute neccessity at times)... However, the source of the pain is still there. Meantime, perhaps (or at least this is the gossip I've heard about painkillers) they have drugged/slowed the musculature in the GI tract so that things have slowed to a crawl. That means you are building up in spots where you have massive inflammation, scarring, etc.. All that extra material is now in places where normally things have passed thru in a hurry, and not put any strain, pressure, what have you, on these problem areas. Then, the drug wears off. Can you say 'Ouch"? (it's funnier if you pronounce it O'****).
Can you see the beginnings of a never ending viscious cycle? You take pain meds, they work wonders while they last; then the pain all comes back, only worse, when they wear off... So, you take more, and then more... Well, you can see where I'm going with this.
I used to live... literally count the seconds.. from dose to dose of my pain meds. Almost got into trouble with them. Sometimes, feeling the pain is a better way to go. Sometimes...