How do you know that? Not all coffee has the same amount of caffeine. The caffeine content can actually very widely depending on a number of factors, such as the type of beans, the roast, the amount used in brewing, the length of brewing, or even the temp of the water.

Caffeine is a powerful stimulate, the response you indicated is not at all out of the ordinary. I myself have to limit caffeine after dinner or it will keep me up all night too. The half life of caffeine is stated as 5 hours, so that means after 10 hours 1/4 of the caffeine is still in the body.