It could be you're running out of RAM.

You could try typing free or free -h in a terminal to see how much RAM you're using. If you see swap as 0, then it means you don't have it set up or enabled.

I don't want to waste time explaining swap space if that's not your issue, but if it is then you can probably figure it out with some searching or come back for help once you've confirmed that RAM is the problem.