As the man said.

It could be that your BPD is reacting to what you think is an unfair statement of fact, ie we observe there is a problem we suspect you are Not using a drug as prescribed so we are going to ask you to take a test and rule that out.

It could be something else but in any case your best option is to return to IOP and work it out. The fact is they were open and above board by telling you exactly what they thought and what they wanted. That should be a taken as sign of respect becouse they could have easily said we want to get a blood test for x but tested you for oxcodone.