Can UTI Medications Cause Black Stools?

I was told i had a UTI on Friday the 7th of February so the doctor prescribed Bactrim twice daily for 3 days but now it's the 13th and I'm done taking those pills and I have a black stool. Also I didn't want it to come back so I took a Cipro on the 10th and one yesterday. Tell me what I should do or what's happening to me!