I had nyquil cold an hour after i had green tea, later that day i started seeing white and felt dizzy, i almost passed out, what caused this reaction?

Nyquil: There are multiple medications in Nyquil. The decongestant in Nyquil can cause increased blood pressure which may have caused your symptoms. Nyquil also contains significant amount of alcohol and if you are used to drinking alcohol, this could have caused your symptoms. No doubt the symptoms you had were due to the Nyquil, and you should avoid this medicine in the future.

Most likely a drop: in blood pressure was the immediate mechanism, however, yet the reasons why this may have occurred are many. Blood pressure is constantly being adjusted by our brain via adjusting venous tone, heart output (via rate & intensity of contraction) combined with "total peripheral resistance" (average degree of dilatation of our 20,000 our more end arteries called arterioles), water intake/output, etc.