What Does It Mean When Your Nose Bleeds And You Didn't Even Do Anything To Make It Bleed, You Were Just Sitting There?

No, don't be worried, but pay attention to that and the frequency of them.   They may occur due to sinus congestion or pressure from a small broken blood vessel.  It isn't the fact that you had a bloody nose, what you want to watch is how often you get them or if they will not stop bleeding.  My son would get nose bleeds often and they finally cauterized a vessel in his nose, and he never had another one.
When your nose begins to bleed, lean forward and pinch the top of your nose slightly.  Do not lay your head backwards or the blood will run down your throat and could make you nauseated.

Most of the times you bleed because of dehydration you need to drink plenty of water..

Maybe you have high blood pressure.
Or you've been indoors to long were the surroundings are to dry.

It is nothing to worry about, it happens to one of my grandsons all the time. Just for your peace of mind, you should talk this over with your doctor for some tips and ideas.

Some times dryness will start a nose bleed.  Make sure you have a window open a bit in the house. Hot air from the heat is not too good for you. Some people keep a pot of simmering water on the stove to raise the humidity. Then sometimes if you pick your nose (and we all do) you could scratch yourself with a finger nail.  Always remember. "You can pick your friends and you can pick your nose, but you can't pick your friend's nose."

Yes that isn't a good sign your dog should not get nose bleeds contact your vet asap

It can mean that you don't have enough vitamin c...I think. 

Usually, it's cause of the nosebleed spook, He starts it and then him and his friends sit back and laugh