I had at least 6 concussions within a year only 3 were major.

My question is are you sure that you did not bump or jostle your head again?

My last concussion was not a bump, but I was sledding and just from going over the bumps I re-injured my head.
I didn't think I could call it another concussion because I thought that meant only when you hit it, but actually just as much damage can be done through intense jostling.
Jumping rope, jumping and landing hard, falling, slight bump into a cabinet door, anything that touched your head or caused a vibration up to your head could be the cause of another concussion.

I hope this might give you a clue as to the cause, for me that was one of the major things, knowing I wasn't weird, knowing that something out of my control caused it, and knowing that there was not much I could do to help it except try not to hit my head again.
Patience, that is the thing I have tried to learn...hard lesson!

Good Luck
Merry Christmas!
Margarite