Omnicef and stress

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.

Omnicef is actually one antibiotic my 2 year old has little trouble with. The red stools are alarming but at least they are solid. Currently he's on Augmentin which has caused horrible rash and loose stools.
The other nice thing about Omnicef is the dosing schedule.Usually once or twice daily depending on the infection. The 3x daily with Augmentin is way too much stress for me to bear,and for him even more so. Seeing his poor little red butt makes me cry with him. Since the two drugs can be used for the same infections, I will be asking for the Omnicef first from now on. Omnicef seems to clear his infections very quickly. The Docs really should forewarn about the red stools though, they are quite scary,even though they are typically harmless.