Why did I react to benedryl like this?

I receive medication at the hospital once every two months, and they give me benedryl beforehand to prevent an allergic reaction. Usually they give me two small pills and it makes me pretty sleepy, but it happens kind of gradually and not to a ridiculous extent. Yesterday, however, I was given one larger pill. When I questioned it they said the dosage of the one pill was equal to that of the two smaller pills I usually receive. But it hit me like a rock! I got very tired, VERY suddenly, felt like all my limbs were extremely heavy and fell asleep even though I was trying not to. Why would this happen if the dosage is the same? Was it released or absorbed differently because it was one pill instead of two?