Blood tests are usually run to detect medication levels in the blood, and are often used to see if the level registering for each drug are in the "theraputic range". Pain meds are a little different, in that the patient has to tell the doctor how well the pain meds are working, so the doctor can compare that information with the level of the medication showing in the test results. Blood tests also show if other substances, i.e., drugs not prescribed to the patient, illegal substances, are present, which could seriously affect the doctor's future treatment plans. Doctors basically want to verify that a patient is receiving the appropriate levels of medication and is sticking to the rules. If there is a serious concern about the patient's drug intake, the doctor would most likely take a hair sample for analysis.
As for fluctuating the amount of pain meds taken, doctors don't have a problem if a patient takes less medication, but are much more concerned if they are clearly taking considerably more than prescribed. A few pills here or there isn't a huge deal, but if said patient is going through the meds twice as fast as prescribed, and hasn't notified the doctor that there is an "increase" in pain level, then there's going to be a problem.
Honesty is always the best policy, and unfortunately, the boom has swung to the side of paranoia in the medical community. Nobody wants to stick their necks out anymore, and doctors that do still prescribe narcotics/opioid pain medications are, for the most part, much more hesitant to do so than in the past, and are far less trusting than they used to be. People have abused the system to the point that life is far more difficult for people like us who have to deal with all the new hoops they want us to jump through. It's not fair, but we have to be our own advocates these days. No one is going to do it for us.