May 10th i started taking nitrofurantoin for a uti today will be my last day of taking the pills but there is still light blood in my urine? no burning or pain when pee i did take a plan b may 7th which did make my period come on early for 4days?

You did not tell us when did your period start after taking plan B because blood from menses can contaminate the urine. However, if other symptoms of cystitis resolve, only pink urine or urine with streaked blood remains, you should be fine after 5 days of nitrofurantoin.

Collect urine in a way that your vaginal contents dont contaminate it and have it checked. For there is a possibility is that your urine specimen was contaminated by your period.