First, watch when you take the Vit. C. I know that
orange juice and other citric acid drinks can hinder the absorption of stem meds. Not sure if a pill would do the same.
In regards to your question, my only thought is it just could be the timing. Extended release meds do just that. They release at different times of the day. Possibly, you were unlucky enough to have been tested just shortly after a dose hit your system.