Omnicef and medicine

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.

My 4 year old daughter was on omnicef for two days for an ear infection. She is now covered from head to toe in a nasty red rash. The rash on her cheeks looks as though the bumps have all meshed into one huge red bump. Her entire cheeks are bright red. The dr said it was an allergic reaction to the meds and changed her to Septra. I don't want to start her new meds until the rash is gone but it isn't showing signs of improvement as of day 3. I am wondering if anyone could tell me how long the rash lasts? Should I start her new meds for the ear infection anyway? And she is sooooo moody. Her meltdowns are on a nuclear level which is out of character.