Eventually with long-term use of opioids it gets to a point where your brain just doesn't respond in the same way anymore. The high is never as good as it was in the beginning, even with long breaks. I don't think it was directly from using low doses of methadone for 1 week a month ago, probably just from using opioids in general. As Mass08 said, tolerance doesn't really reset back to zero once you've used opioids for a while, even if you take a break.
There could also be other factors though, for example do you use any other drugs?
ETA: You should always go on the assumption your tolerance could be low after a break and start with a low dose. You can always take more if it's not enough but you can't undo it if you take too much. Taking 8 Vicodin (to get 40mg hydro) would also be a lot of APAP, that's 4000mg at once. It's not wise to exceed 3000-4000 over a 24 hr period (depending on the source); taking 4000 at once is not a good idea.
