simply because some of the opioids attach to your nerve endings, the more consistent the use, the more they make themselves at home.
when you stop, and the opioids leave your nerves, they are in a sense reexposed to the environment, and shocked.
also, your body will start running faster, warning up, you begin sweating, your pores open to release body heat, or the may think they are supposed to, as they should if you had a virus or sick. when your pores do this, you take in more cooler air which is uncomfortable anyway, and with your nerves being hyper sensitive it feels much worse.
i try and picture all this microscopic mayhem as its occurring, see it for what it is, and detach some into this physiological visualization of electricity, highways, and homes. its easier then fighting it, or trying untill its run its natural course.