Can antihistamines like Zyrtec cause dry cough bec I think my post nasal drip may be gone but still take Zyrtec when I think it possible may not need?

No. In fact Zyrtec may help control a dry cough when the cough is caused by post-nasal drips. I note that you have reflux as well and this can certainly cause a dry cough. Cough-variant asthma in which cough is the only symptom is another potential cause since allergic rhinitis are often associated with asthma.