The doctor looked under the microscope as yeast. But the test came up it wasn't. Symptoms are yeast except its burnin a little what is that?

Yeast infections. Vaginal yeast infection and vulvitis cause symptoms that are nonspecific, which means that aside from the yeast infection, other conditions can cause the identical symptoms. The most common symptom of a vaginal yeast infection is itching in the vaginal and/or vulvar area. Other symptoms include:burning, soreness, pain during intercourse and/or urination, and whitish discharge.
Probably not yeast. Unless doctors look at hundreds of samples it is easy to make a mistake looking under a microscope. The lab pathologist or microbiologist looks ad hundreds, thousands and rarely would miscall a sample unless the sample had not been properly handled. Sometimes other organisms, like a virus or infection of another kind will cause burning like you describe.