Don't they just dip a stick in the pee and if it turns a certain color they know you have a UTI? If so, I don't think you have anything to worry about. If it shows up, just show them your prescription bottle. It's not as if you're scoring crack on the streets. It's a legitimate medication.
Not a criticism, but just a random thought: perhaps if more people acted like phen is what it is: a legally prescribed medication,- more people in the world would get over the "she's doing drugs" mindset. I went out of the country recently and just put my bottle in my pint size baggie of stuff for security. No problems getting out or in of the U.S. or St. Martin with them. Ditto with the Xanax and Doxycycline. No questions asked. Of course, I just took along a couple of days more than enough for the time we would be gone. I'm paranoid about airlines losing stuff- even meds that are in my possession at all times.