It could have nothing to do with physical symptoms, but be a mental one.
It isn't too common, usually those who use heroin get used to it - they start to love the smell, love the taste, that kind of thing.
But some people have a strong psychosomatic aversion to heroin. They will puke will prepping the shot, they puke if they even smell the heroin by accident, they will puke their guts out hardcore just thinking about heroin.
Or it could just be me.
But I have, on many occasions, puked just from thinking about heroin. Usually it is much worse if I have been doing heroin recently, and if I do heroin daily, after about 4 weeks of daily usage I will puke at anything related to heroin. I have, on many occasions, puked at just looking at brown soup just because it has reminded me of heroin. Usually it doesn't get this bad, but after shooting up daily for about 2 weeks, I will puke every single time I go to prep a shot. I will be perfectly sober, but just dealing with the heroin sets something off in my mind, and I may very well puke extremely hard and wretch for a while before I can finally finish making a shot.
If this is what is happening with you, I can't really offer anything to help. I've been shooting heroin for over 2 years now and I still have this problem.