Help with a diagnosis? (Post Nasal Drip Related)?

I was told I have post nasal drip by my doctor, and he gave me some nasal spray and told me to take some antihistamines,robitussin,and zyrtec. I did this and haven't felt better. I've had this post nasal drip for around a month, but this week it has seemed to kick back up again to a real bad stage. The constant drainage is causing my stomach to hurt, but I'm not sure if this could be a sign of IBS, Acid Reflux, or just the plain drainage? Help anyone?