This is, quite specifically, why Apple never refers to these machines as "laptops." The term used is always "notebooks."
The lower left quadrant of most MacBook models is where the hard drive resides. It quite naturally gets hot when it is used heavily, but what you are feeling is the *heat being dispersed* by the metal chassis unless, of course, you are preventing that heat from dispersing well by laying the bottom on a soft, heat-absorbing item such as a thigh or a pillow. This effectively blocks the machine from dispersing the heat properly, causing the fan(s) to work much harder/noisily to compensate.
In short, everything sounds normal.