The PM is under NO obligation to provide you with a last prescription. They can and usually do dismiss you for a failed urine test.
Was the Oxycodone you took prescribed for you?
It would be highly unlikely that fentanyl would have shown up in urine if you haven’t taken it.
They are two very different medications-and fentanyl has a very specific test that must be requested to look for it.
I think they can kick you out right then and there particularly because she gave a warning. Unless there is a plausible reason for the failed test. Have you mixed and mingled at all that would lead to that? If you never failed one before and aren't doing anything different, I don't know why you would now. Talk to them about it. Make sure to come back and tell us what happened. They are so strict now with drug management programs. I guess for liability sake but it leaves a lot of responsible med takers in a rough space if a rule is inadvertently broken. If you are are using something other than you should or using what you have incorrectly, act on it to get help NOW before it is a bigger problem. You sound reasonable to me and I hope it all works out. Let me know.