Why Did I Die That Night?

Some kind of ecological event caused hundreds of thousands of tons of CO2 to be released from the nearby Lake Nios. Some have said it was an earthquake that triggered the release, but neither myself nor my father remember any kind of tremor the night before. Because it's heavier than oxygen, it fell to the earth and covered all the surrounding towns and villages for miles around. I still have no idea what caused the wounds in my arms. And although humanity now understands a lot more about the incident, some things still remain unexplained to me. This called it an event which occurs just once every thousand years, and that we were just unlucky to be in the area on the night in question.