My 3 year old and my 19 month old are on Omnicef for strep throat...

My 3 year old and my 19 month old are on Omnicef for strep throat. Both have been cranky since starting the antibiotic. My 3 year old son is who is usually very happy and easy going is a monster. Throwing things, slamming doors, disagreeing with everything. My 19 month old daughter has been having a lot of trouble sleeping and has also been very cranky. They took their last dose this morning and right before lunch my 19 month old broke out in a rash, starting on her legs (looks like tiny pimples). When she woke up from her afternoon nap, she seemed to be constipated. She was straining and contoring her body like she had cramps. She was crying and straining so much I though she was going to vomit. We put her in the tub to see if a warm bath would help with the cramps. She rolled around in the tub in pain (crying the whole time). When we took her out, she didn't want to be touch. She didn't want anything to touch her skin (not even a towel). While we were on the phone with the nurse line trying to decide if we should take her to the emergency room, she stopped crying and decided she wanted a fruit cup. This is some bad stuff. Avoid it.