What causes paradoxical reactions to benadryl (diphenhydramine). what mechanism ocurrs in the brain that makes the person stimulated rather than drowsy?

Not entirely known: But you're describing a very real phenomenon. The same can happen with pain medicines and other sedatives too.

Unknown: I've never read an explanation. I'm sure there is one published but i doubt it's proven. It tends to happen more in children thus suggesting that there is inhibition of a disinhibitor. (i.E, mumbo-jumbo!). More to the point: if it happens to you, avoid further use.