Please see your doctor as soon as you can. It could be anything from a bug bite (the problem is, what type of bug or spider bite) to other problems due to circulatory problems within the leg. If you are 35 years and younger this is highly doubtful. If you were out drinking the night before or even playing sports you may remember hurting yourself.
It's best to have it checked out by your doctor. Meanwhile, bath it with epsom salt, put a clean bandage on it to keep your clothing from rubbing on the scab.
Usually a scab means you are healing, but unless the bruise is starting to turn yellow (a healing sign) get to the doctor.
Its an impact wound that was hard enough to break the skin. It should go away in a few weeks. You could have hit a coffee table and not remembered it, or sleepwalked. The scab should fall off in a couple days. Dont pick it off, its natures bandaid and keeps the wound from any infection.