opiates can cause nausea, cigs also can cause nausea, the combined effect can make you feel sick even if the individual amount of each one wouldn't be enough to make you sick on its own.
For me I only get it if I smoke a lot of cigs in too short a period of time. like if I smoke 4 in an hour(might not be that much for pack+ day smokers but I only usually smoke a few cigs every couple days but when I am high on opiates I smoke a lot in that time period, but like I said 4 an hour or more I get sick so I keep it to a cig every 20-30 min while high