I was given omnicef for a sinus infection, though I was told to...

I was given omnicef for a sinus infection, though I was told to take it twice a day , ive only been taking it once a day , due to an unnormal schedule... It has caused me to be so uncomfortable..I cant stand it. I have unbelievable abdominal pain, cramps ... but no diarrhea... Im constipated , one bowel movement a day ... and I also have the rust/ red stools. which is a listed side effect . Ironically, Before being diagnosed with all of this, I was in the doctors office a few weeks earlier complaining of stomach problems. . thinking I may have an ulcer, and my doctor told me It was only stress... yet he never tested me for an ulcer, no x-ray , nor an endoscopy. Now Im on some medicine that apparently may mess me up even more. WOW.