The caffeine in coffee works as a stimulant primarily by blocking a neuromodulator called adenosine. In a normal waking cycle, adenosine gradually accumulates at certain synapses in the brain and in doing so will contribute to increasing drowsiness. But caffeine acts as an adenosine antagonist, blocking the sites to which it binds and preventing it from working — the result is a feeling of increased alertness and energy which begins within a few minutes of consumption and continues for several hours. However, the body will tend to produce more adenosine to compensate for the fact that adenosine doesn’t seem to be working under the influence of caffeine; this means that once the caffeine wears off, increased drowsiness is likely to result. This ‘vicious cycle’ is responsible for the addictive quality of caffeine, and withdrawal for a very heavy and regular caffeine user can be most unpleasant and even medically dangerous in some cases. Your coffee may be heavily sweetened, and the drowsying effects of the associated sugar crash may be stronger than the opposite effects of the caffeine. There is some evidence that stimulants affect persons with ADD/ADHD much differently than others, though this distinction is mainly applied to a family of stimulant drugs which are dissimilar to caffeine, and your reaction to caffeine should not be taken as a marker for such disorders.
Caffeine does seem to affect different people in different ways — but caffeine does not chemically induce drowsiness as a primary effect and would never be used in such a way by a physician, for example, so it is probable that something quite funny is going on if you find that even large doses of caffeine routinely make you sleepy. Caffeine, in my opinion, could possibly be a chemical which opens up airways because it shares a mechanism of action with a type of asthma medication: theophylline. Both block degradation of a protein (cAMP) which leads to the cellular effects like smooth muscle relaxation of the lungs and airways. This may also be why caffeine may lead to an increased ability to smell. Now I have two speculations for the sneezing aspect. The first is that during this period where airways are open, more irritants can enter through your nasal passageways and cause sneezing. The other possibility is that since caffeine is a relatively weak opener of airways (unlike the asthma drug I told you about), it probably wears off fast leading to a rebound effect which is quick constriction of the airways (which will happen quicker the more your tolerance builds up) which leads to irritation causing sneezing. Some people who are diagnosed with ADHD and Bipolar will not experience a stimulating feeling when drinking coffee but rather a calming effect. She further explained a experiment paper showing that coffee had a calmative effect rather than a stimulate “wired” feeling when tested with a group of ADHD and Bipolar patients compared with their regular and placebo group.