Why Does My Bruise Have A White Line Down The Center After A Hard Knee Impact On My Lower Leg?

Hi, yesterday we were doing some clinching exercises and I got kneed on my lower leg quite hard. It is obviously swollen, but there is a strange development. There is suddenly a white line going straight down the middle of the bruise separating it kind of into 2 halves. I have no idea why and the white line was not there yesterday. My pain tolerance is quite high, so I m not in pain, even though the bruise is the size of my hand. (Majority of people would likely be in pain from this kind of bruise). I just want to know why there is this white line going through the center of the bruise as I ve never experienced this before and I ve had MULTIPLE bruises... Thanks in advance.