That behaviour is expected, as each updated port needs to be compiled for your system. Some ports are simpler and don't take much effort. Others are more complex and will cause your MacBook to heat up as it works (example: ffmpeg).
Given you say you haven't updated your ports in a while, likely there are quite a few ports that need updating. You can take a look at those with:
From there, you can choose which ones you want to update, if you don't want to update all of them:
sudo port upgrade PORTNAME
Additionally, you can see what MacPorts is doing by looking at the output in your Terminal. Typical steps where your MacBook may heat up are:
---> Configuring PORTNAME
---> Building PORTNAME
Do not be concerned about the temperatures. If they really become too hot, your MacBook will shutdown by itself to prevent serious damage.