Sick kid, to be or not to be, or should I send her to school?

What is the correct degree of sickness, where it is appropriate to keep your child at home?  I have no idea.  I always thought, well, it is all about the way they feel, but today I sent my daughter to school with a stomach ache, I am sure she wasn't feeling that great.  Should I feel guilty???  Is she going to be alright?  I am sure she will be fine.  She did not have a fever, nor was she throwing up, although she did spent most of her day in the bathroom, yesterday.  This is what I figure, if it does get bad, she will go to the nurse and they will send her home, I mean, its school, not a labor camp, right?