External Monitor Issues

I have a 2015 Macbook Pro and I have connected an external monitor via HDMI. Everything was working fine until one day I tried keeping one full-size application on my monitor and switching between full-size applications/screens on my Macbook with the 3 finger gesture. However, instead of just the Macbook switching between full-size apps, my monitor also switches apps/screens at the same time. I've tried to disconnect and reconnect as well as arranging the monitors via the display settings, but I've had no luck. Everytime I want to switch apps/screens on one screen, it does it for all.