How To Cure A Large Bruise On Leg Due To A Fall?

I recently fell at work and sprained my knee (5 Days Ago) and my lower leg between my knee and my ankle in the front has been sore and tender to the touch. Now just within a few hours a large red and purple bruise has appeared on my leg. Is this normal? I haven't hit it on anything so I don't understand why it would just appear out of nowhere almost a week later..