Someone is not telling you the truth.
Exalgo is simply a brand name for Dilaudid aka hydromorphone. The drugs are exactly the same. Again, please see the link above.
The other thing that doesn't wash is that, if this was a urine drug screen, they do not test levels, they simply test for the presence of the drug.
I would be on the phone with them, armed with the link above, and tell them what I have told you and then demand an explanation.
If it turns out they did a more specific test that measures levels then yes, taking Exalgo the night before would cause a relatively high level if you were tested in the morning.
My bet is that they are not telling you the truth for whatever reason.
As a rule pain physicians, and regular physicians, do not like prescribing Exalgo/dilaudid long term because of it's abuse potential, it's addictive properties, and it's very high street value so someone may be spooked or under investigation by the FDA and they may be cutting back on their narcotic prescriptions. This is, at least, a likely scenario.
standing by if you have further questions