It could be that you have gotten used to the amount of caffeine you consume and your body now requires more in order to get the desired affect.
My body has never been effected by caffeine, the only times it does effect me it makes me lightheaded and uncomfortable.
How often do you drink caffeine though? Your body could be putting up a tolerance to it.
mayb ur body got used to the amnt of caffeine u consume try to increase the dose