Since you don't remember any trauma, please see a doctor about this to have it checked for melanoma. You can get a melanoma there, and it looks like a bruise.
Well, you might lose your toenail. Did you injure the nail at some point? I kicked something and it hurt some then I got a bruise and then a little while later the nail started to fall off. Another once came back in but it took a long time. If this happens there is nothing you can do to stop it.
you probably hit it or smashed it somehow. your toenail may fall off ifso bandage it and itshould be fine. I woud mention it to your doc though could be a clot.