My layperson's understanding is that the opiates remove the pain and eventually, the person's body re-calibrates its interpretation of pain based on this new state. When they stop with the opiates, normal levels of pain return, which are above and beyond what they're prepared to handle and they'll do nearly anything to make it stop.
One time, I heard a recovering addict talk about his hair hurting. He had been numbing himself for so long, the movement of the hair on his scalp felt agonizing to him.
Based on my conversations with friends who were/are addicted, that seems about right. After stopping long term opiate use, your sensitivity seems amplified. That's why addicts who can't get their fix are often seen scratching themselves - they feel itchy everywhere.
This is part of it, but not the whole thing. Withdrawals will involve pain, but also depending on how much and how long someone had been taking opiates, you will also experience: physical flu symptoms, insomnia, anxiety, induced restless leg syndrome, and more.
One time, I heard a recovering addict talk about his hair hurting. He had been numbing himself for so long, the movement of the hair on his scalp felt agonizing to him.