What Causes Lortab And Xanax Not Being Shown In The Urine Test?

i am on lortab and xanax...my doctor did a urine test and first the lortab did not show up...so they did a second one and this time the xanax did not show up....and i take these bills several times a day. now my doctor will not fill them anymore cause she says since they don't show up in my system...i must be selling them. i am SO upset!!