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.
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.
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.