LONG DIARRHEA

  dear doctor,

I am a 32 years old.I never went to a doctor for any type of disease.Yet starting from January 2009 i have had  diarrhea.In the end of January I did a check-up of my stools.They said that I had Giardia parasite.I was given Flagyl 500 for 15 days -bedelix-smecta.Yet after the 15 days the diarrhea came back again.
I have no other  symptoms associated with it such as blood, vomiting,fever or any thing else--only frequent daily diarrhea.
  My diarrhea is somewhat yellowish.sometimes -not very often liquid- in other times it is loose. i also notice food parts in my stools.
  Can any one give me some advice and information about this problem ,please?
  Can Giardias persist after 15 days of antibiotics-Flagyl 500?
  Or is this a sign of an other problem?
  thank you so much for your help?