This has happened to me - finding bruises when I wake up and don't remember getting. My doctors said it hapened most likely from being in a dissociated state.
What I would try to find out as soon as possible is if you have some condition that makes you bruise easily. If you are on medication, it is possible to have that kind of side effect and would explain why you don't quite remember getting them.
If this is happening while dissociating, you could try to stay grounded as you aren't hurting yourself in that state. Here is a link with some ideas on grounding techniques that you can use:
http://www.recoveryourlife.com/forum...g+techniq ues
Take care