Well, yeah, you probably should've taken her to the dr. before now.
BUT, what sort of cough is it? Is it a dry, hacky cough? Or is it a barky cough, sort of sounding like a seal? It could be croup, or it could be whooping cough. There have been lots of reported cases of whooping cough going around.
Does she have any other symptoms at all? Is her nose stopped up? Are her ears plugged up?
With our 5 year old, recently she had a bad cough, and the dr.'s said they didn't like giving cough syrup any more because if the cough was being productive and getting mucous out of the lungs, then the kids got better faster.
Also with our 5 year old, she gets asthma-like symptoms sometimes when she's sick. We have a nebulizer and can just about recognize when she needs it anymore.
Regardless, only the doctor can really tell you what it is your child is sick with, whether it's whooping cough, croup, an ear infection or sinus infection, whatever, and help prescribe something for it.
Good luck!