This has happened many times now, especially since the new version came out. Health reaches zero, or even below zero, but that does not show. As long as the last creature that atttacks is antimattered your health is restored to above 0 and you can keep playing. For some reason it doesn't always happen, but quite often it does.
It does not have to be anything with poison either. It can be: 10 health: 5/5/5 damage (You're dead) then -4 from antimatter, you would still be dead, but you simply have +4 health.
Very odd.