I Have Had A Upper G I About Three Years

I have had a upper g I about three years ago and I was doing the one day fasting for my church and for the last three weeks I have diareara after everything I eat. I take two stomach pills one in the morning and athe other one at night. the one is for my alser and the other is to keep the acid heart burn down. but the last couple of times going to the batyhrrom my stool is has been a blackish color. why?