The 200 day implant lasts longer than 200 days, 200 days is just the amount of time it can be assured that it will provide what is determined to be an adequate blocking level of naltrexone (enough to completely block the effects of a specific amount of heroin usually), so it's possible you still have residual naltrexone in your system. It's not like it just stops doing anything after exactly 200 days. It would still have extra naltrexone left, plus naltrexone can take quite a while to leave your system as well.
It is also very common for people who have been dependent on opioids to have decreased effects from opioids even with a substantial period of time without using them. Especially if you are expecting to get effects from a low dose of a weak opioid (not sure if that's the case with you). But my bet would be that there might still be naltrexone in your body, at least that's a possibility - you won't really know until you get the implant out and wait enough time for any naltrexone to be eliminated from your system.