I'm noticing a pattern here

Two days ago I drank only green tea. I felt terrible all day.
Yesterday I had two cups of coffee in the morning at Men's Fraternity at church. Then I had two glasses of the nectar of God, I mean, Dr. Pepper, at lunch. Felt GREAT, if a bit jumpy, all day.
Today I had a cup of coffee, mostly decaf, and a caff-free Dr. Pepper for lunch. I feel about 40% as great as I did yesterday.
A pattern is emerging.