Omnicef and flies

Our son has a non-allergic rash to penicillin at age 1, so he was put on zithromax to cure his bronchitis. Since then, he's had no antibiotics. Until last week, at age 6-and-a-half, when the doctor diagnosed an ear infection. He's been on Omnicef for a week, and has been very irritable! Flies off the handle at any moment, growls at his siblings... he has just generally lost the ability to "deal" with things. Our younger son has been on Omnicef twice for ear infections, and has had NO adverse reactions to it, other than some very loose stool, which is almost expected. I guess it works differently for each person!