In the protank heads (towards the bottom of it) is a coil of resistance wire which heats up and does the actual vaporizing. The heat dissipates into the rest of the
device and the air, but if you do it a bunch in a row, you'll start to noticeably warm up the surrounding metal on the device. It's supposed to happen (and happens even more noticeably on some devices than others).
Maybe try not to take more than a few drags immediately one after another without tossing in a 15-20 second break at least.