What are you guys talking about "there's no reason to shoot up after being clean so long"?... That makes precisely zero sense. Hah. IV'ing is exponentially better than any other ROA, especially with something like Heroin, which comes with the amazing rush you only get when you shoot up. Most of us serious users, who have made the jump to IV'ing, (whether we initially did it because our tolerance was high or whatever the reason), can not go back to snorting or smoking hah. That's like going from having sex every day with a supermodel, to jerking off alone in your basement... :-/ Yes, though it can be dangerous because your tolerance is much lower, you just do a lower dose to start off. Hah, but you don't snort it or smoke it. Most of us, including me, could have three bundles of prime dope sitting on our dresser, but if we don't have any rigs at the moment..we wont just snort the shit, no matter how sick we might be. It's looked upon like a complete waste and if you're forced to take a tiny bump because you're hurting so bad you can't even make it to the drug store, you're pissed that you had to even waste that much. You wait until you can pick up a rig, and then go to town... So, unless my ten plus years of being a junkie and spending time with other junkies has failed me..this is how it is.
As to the OP's original question, chances are the dope was just crap, or you didn't do enough. There have been times where I expected my tolerance to be way lower than it was because I had been clean for a while. But the fact is, after so long of using..your body will never dip below a certain tolerance, at least not for quite some time. You'd be surprised what the body and mind remembers even after a year of not using. It's still the smart move to start of slow anyway your first time back but, your tolerance probably isn't as low as you think it might be.
Anyway, thats my two cents.
Good luck.