When was your last dose taken before the test? 5 mg is a tiny dose, and I believe the test needs to be specifically requested since it is a semisynthetic opiod, not a traditional opiate.
Although oxymorphone is much more potent than other opiates, if you drank a lot of water, urinated frequently prior to the test or it wasn't sent out for confirmatory testing, it is possible an in office screen could show false negative.
The withdrawal symptoms can be managed using over the counter remedies, imodium, gatorade, and hylands restless legs are typically suggesred.