right now on clarithromycin,been on for 4 days twice a day 500/mg.i have a sinius infection.and this medication does cause upset stomach.this morning i had bright blood in my stools, this on his not common with me,until after taking my medication..what is happening,what should i do... I am a white female/age 52 yrs old,height 5'1/weight 165..i took something for my upset stomach ,prior my passing blood with my stools