I was diagnosed with sinusitis, bronchitis and tonsillitis when I...

I was diagnosed with sinusitis, bronchitis and tonsillitis when I was prescribed Omnicef 2 weeks ago. I began taking it immediately. Terrible black diarrhea began the next day continued during the entire course of antibiotic and now after finishing the medication four days ago I have persistent diarrhea that is clear with some bile. This has never happened to me before and now I am very concerned after reading everyone else's reaction. Why is this stuff being prescribed??? Sorry if the descriptions are too graphic....