I have been prescribed 50mg nitrofurantoin for 3 days (12tabs) for a uti. i have already taken 4 and feel no different, is this normal?

Yes, but 50mg is low: Symptoms may take 2 - 3 days to subside. Most dr.S prescribe 100mg 3 or 4x a day, or Macrobid 100 mg twice a day. Need to get urine rechecked if symptoms persisit more than 3 days because infecting organism may not be sensitive to macrodantin. In which case another antibiotic like Bactrim or Cipro (ciprofloxacin) may be necessary. Suggest urine culture & sensitivity test is performed if infection still present.