Why Does Dilaudid Show Up Negative In Urine Test Despite Taking It Daily?

I have tested negative for Dilaudid/ exalts two times. Both test were oral. I m in trouble with my pain mgt. Dr. He did a regular urine test today and am waiting for results. Any reason behind why I ve been negative? I take 1 16mg Exalgo 2 x daily and 3 8 mg Dilaudid daily. Please help.