I Take MS Contin 30mg Q 8hrs For Chronic Pain.

I take MS Contin 30mg q 8hrs for chronic pain. Along with endocet 10/325 q 8 hrs prn for break thru pain. I also take carafate tid/ zofran q 8hrs prn/ bentyl q8 hrs prn/synthroid 0.1mcg qd/ nexium qd. I take monthly drug tests at my primary Drs appointment. The last test didn't show my endocet but it showed hydrocodone. How can this happen? It's making me crazy trying to find out why. Thank you