If your heroin had fentanyl, that's going to be tough. I'd be hesitant to reveal your usage unless absolutely necessary, there are unfortunately some very biased doctors who think they're morally in the right to deny adequate pain relief to drug addicts. If the heroin had no fentanyl, you'll probably be pretty uncomfortable still, but manageable. You won't be able to hide the withdrawal symptoms from the nurses and vital checks, though. Any chance you could just ask for an increase in pain medication, they usually start off pretty low for patients who they believe are opioid naive.
Edit: I missed the part about 2 mg iv every three hours, you may fare better with that, again depending upon fentanyl.