Pain Management & Urine Drug Screening?

I am currently under the care of a PM doctor. Upon my very first visit as a new patient they did a urine drug screening which is fine because I don't use drugs. I had already been on hydrocodone from the doctor that referred me to this PM doctor and I told them upfront what I had been previously prescribed. I also was honest with them about the dosage per day and how some days I don't require what I'm allowed to take, but then some days I may need more, but that by the end of the month it all evens out because I don't run out of my meds. Anyway, I had my regular monthly appt. yesterday and they did a urine drug screen on me, which is the first one since I started going to them which was 9 months ago. My question would be - is this normal? Do they perform random drug screenings? I had to fill out paperwork and tell them what I had taken and the last time I took it. I've been sick the last couple of days and hadn't taken my prescribed meds in 2 days because they made me nauseous. I guess I had a virus. Will that flag my urine test as not having any of the prescribed drugs in my system, or does hydrocodone stay in your system longer than that and I'll be ok? I'm not really worried because I don't take anything illegal, nor do I take any other type of pain meds from any other doctors unless I notify my PM first. Their policy is to inform them if I have to undergo any procedures for anything and that the other doctors can prescribe pain meds for me - I just have to inform my PM doc and he adjusts the pain meds he gives me - like he'll give me my prescription, but maybe I won't be able to fill it for another week, which is fine. I'm still not really schooled yet on how a PM doctor/clinic works. I had some extensive oral surgery a couple of months ago and I let my doc know at the appt. I had before my surgery and he notated it in my file and I also told him what I had been prescribed for pain. I also told my oral surgeon that I was in Pain Management. I'm also about to have some knee injections and have informed my PM doc about it and I've told him that I'd most likely be given a prescription for pain because the dosing schedule will be different from my PM doc's dosing schedule for the hydro that he prescribes for me. Anyway, I guess I'm just asking if the drug screen I got was routine, or if they suspect something. Like I said, I don't smoke pot, or use cocaine, heroine, or even oxycodone. Anybody that has been thru this that can shed some light on their practices would be very helpful. Thank you in advance.