Issue with creating new desktop on secondary monitor when playing full screen video on primary monitor

I'm using 2 monitors with my Mac mini. Prior to today, I could watch a video from youtube on full screen on my primary monitor while the secondary monitor was unchanged. After I restarted my computer today, now when I full screen a youtube video on my primary monitor, my secondary monitor goes black and creates an additional desktop in Mission Control. How do I revert back to the old setting?