Dizzy when laying flat on my back and the alot of post nassel drip?

Acute or chronic?: At your age various structural answers are possible. If this is acute (new) then follow the post nasal drip as a possible clue of a sinus disorder. While this would not directly explain positional vertigo, the two might be related. Benign positional vertigo generally is not associated with nasal drip. Last, a nasopharyngeal leasion might produce true post-nasal drip and vertigo. Get eval.

Possible virus...: This may be possibly related to a viral upper respiratory tract infection which could cause non specific symptoms like the ones you have mentioned. You may also experience cough, fever, chills, nausea, However if dizziness gets worse and you start feeling sicker you would need to be evaluated by your doctor. If symptoms are severe then consider going to the ED.