i have a macbook with retina display and when i use it, it gets hot on the bottom is this normal?

Depends how "hot," but yes it's normal. The computer has numerous temperature sensors, if it truly starts approaching dangerous temperatures, it will begin to slow down the processor speed, speed up the fun, displaying temp warnings, etc.

In other words, it takes care of itself.

It is normal because this machine has no fan at all.

If you're talking about the "new" MacBook with retina display, it's normal (Especially when you're playing games or watching a movie) and like Stark_Hu said, it will start displaying the temperature warnings, slow the processor, and try to regain normal temperatures. I actually had that happen to me while I was playing the Sims 4 while having 3 Pages documents and 5 or so Safari pages open.

Now if you're talking about the Retina MacBook Pro, I would clean out the fan ports and everything because that doesn't seem too normal!

Agree with the rest here. I have the 12" new MacBook and under a decent load, the bottom gets noticeably warm. Not "burning" hot, but warm enough you'd think the fans would kick in, and there are no fans =)