I've experienced this twice so far...the latest being last weekend. I believe I got slammed on the mat when I jumped guard on this one dude. Ref didn't see it. When I hit the mat all I could remember was that I saw colors, everything went silent, and I blacked out for a split second. Then I remember just being lost and holding on to guard for dear life. I felt disoriented for a few seconds. It's like kicking one of those old tv sets...and the picture gets all fucked up. That's how I saw things. Got a headache that lasted till the next day. Was that a concussion? It kinda felt like the day I got flash knocked out in MT sparring a few years ago.