Flu like symptoms went away then stomach cramps, constipation and diarrea?

I am a 24 year old male. 215 lbs (4 months ago). I got sick with flu like symptoms, then those went away. A day later I had slight stomach cramps, constipation, so I did a day worth of a herbal colon cleanse. That didn't work but did give me diarrea, and stomach bloating especially after eating. I've been exercising lightly, and got a slight better diet but now I am down to 202 lbs. I went to the doctor, she played with my stomach and nothing hurt when she tapped or moved it. She prescribed me with a weeks worth of metronidazole. The bloating thing went away, but the diarrea went from liquid to very loose, and my stomach is still slightly upset. Is this a side effect of metronidazole? I recently also Quit smoking(6 year smoker, on 9th week of Quit, which contributes to anxiety episodes) and why did she prescribe metronidazole?