FPS with external Monitor

When I used to play Fortnite on my laptop without a monitor I would get around 160fps. I started using a monitor a while ago and started getting around 160-190fps. The problem is when I unplug my laptop from my monitor and take it other places to play games, I only get around 100fps in Fortnite. Is there a reason for this? I keep my laptop plugged into the monitor most of the time, and rarely unplug it.