Bruising Randomly

Ca
Hi girls I need some help. I've been bruising quite randomly for a while now, I prob should have been taking pictures of them 😒 but I thought it would stop by now. These recently showed up on the back of my knees... really odd place to get bruises. If anyone has an explanation for this please help! 
PS I know they look more red it's because I have the flash on: they are definitely bruises NOT a heat rash or any type of rash.