Discussions that mention amoxicillin

Children's Health board

Here's hoping I'm just a paranoid Mom...But would love some ideas and advice.

Ok...Quick background. a few weeks (about 2 1/2) ago my son and daughter (Emily age 4, Caleb age 5) spiked fevers and were coughing bad...A trip to the ER found Caleb with a small walking pneumonia, and Emily had nothing on the XRay although seemed sicker.

Emily was given a weeks worth of Amoxicillin and Orapred. Caleb was given a weeks worth of Amoxicillin with NO Orapred (although he was the one with the bad xray- go figure)
The next day, my youngest son Luke comes down with a similar sickness. He is given two weeks of Amoxicillin, and seemed better.
Anyway, Caleb and Emily have both been off of antibiotics for ONE week, and Luke finished his dose Friday.

So...A few days ago (Thursday night) Caleb comes down with a fever and coughing again, we take him to the Ped. on Friday, who gives him a week of Zithromax, says it's a sinus infection and Bronchitis. Ped. said fever should be gone by Monday. (Of course being tomorrow)
So...Today, the baby (Luke) spikes a fever, and so does Emily.

What in the world could cause this? How could they get sick while on antibiotics...I'm calling the Ped. in the morning, I am just really nervous.
I realize I am probably paranoid, and have written quite a rambling message, but humor me. :) I'm really worried. I don't know why they'd get so sick back to back like this.
The symptoms are fever (Calebs now lasting over the magic 3 days, not to mention them all being sick two weeks ago) coughing and runny nose.

Help. I'm worried. :(