Please don't beat yourself up over this. Bacteria are EVERYWHERE, and when the preemies are still so young(I've had two), their little immune systems are SO much more fragile. We have bacteria living on our skin, etc. - it is literally everywhere. Diarrhea is VERY common with antibiotics(Augmentin tends to be particularly brutal, but it's a GREAT antibiotic), as is general stomach upset, but as you know, that's far preferable to letting a bacterial infection get out of control. It sounds like you are doing all the right things, these things just happen sometimes. I hope she is feeling better real soon!