How Would Having Trace Levels Of Xanax In My System Affect Me, Given My Legal Prescription?

I had .031 of Xanax in my system and I weigh 235 pounds I was stopped by the police here Houston TX. He took me to jail for a DWI and said my eyes were blood shot red and I wreaked of alcohol yet the blood analysis only showed .031 in my system. I have a legal prescription for Xanax 3 mg a day as needed. How would the .031 affect my body.