when you load up youtube, and when you change resolution from auto 1080p to lower, there is a box that says always play videos in low resolution. Maybe you checked that by accident, however you can fix that when you go on your profile and find settings for video playback or something. I know I found that there and enabled auto resolutions, example when I go full screen it auto switches to 1080p
This usually happens to me if I'm downloading something while my parents are using the net as well. But only if we're all doing tasking things. We don't use cable, so most of our uses require it. It rarely gets that bad though. I hit 240 like, once or twice a month.