We don't use SWIM in Bluelight. Despite of knowing you are talking about yourself or someone you know, it can make posts more complicated to understand.
In regards to your question, there's not much insight I can give you other than that is what happens when you use opiates, and very common when you use heroin but it's also considered a very common side effect from any sort of opiates or opioids.
It's always mandatory that you fast before attending to surgeries and that is because of nausea which is a characteristic, symptom of opiates/opioids intake. Some people tend to get used to it others take more time. After 3 years you shouldn't be experiencing this. It's possible that your stomach is full or that you have drank water, or food.
However, it could also be due to stomach problems or simply because your body is reacting negatively by rejecting the drug. I have also heard from one person that this could be a result from the adrenaline related to the rush. Whatever it is, this is a side effect of opiates.
Perhaps you could consider changing to a less harmful drug or even a milder opiate.
By the way, welcome to Bluelight.