What Causes Irregular Bowel Movements After Having Metronidazole?

I was taking metronidazole 500mg 1 tablet by mouth 3 times daily for 7 days for an infection. a couple of days after the end of the medication I noticed every morning I have a muddy bowl movement in great quantity. I started to take a probiotic to replace any good bacteria. It's been over a week, how long will it last?