because you are way too tired????? :P
I think people react differently to caffeine.... I never lose sleep over having caffeine and my wife can't sleep at night if she has cup of coffee after 3 pm....
Does this happen all the time when you drink coffee?
Do you load your coffee with lots of sugar and milk? I'm wondering if you get a short burst from the sugar, and then experience the let down when your sugar level drops....could that be what's making your feel sluggish or sleepy?
Sugar will boost up your energy and when it comes down it can make your sleepy.... It might not be true for everyone but some people have more effect....