The purpose of a Pap smear is not to detect a yeast infection. However, your doctor should have noticed some inflammation if you have one.
External itching and irritation can be a symptom of a yeast infection, but it's still wise to check with your doctor about the symptoms. Several other factors can cause those symptoms.
As far as a yeast infection, my worst one caused irritation in the vagina, but also external irritation. I had clitoral swelling and irritation, which strangely caused a sort of uncomfortable but arousing sensation. I even produced lubrication.
Make sure to check with the doctor about the problem. Sounds like an easy-to-treat issue, but it's good to make your doctor is aware.