My chief is the one who does potions in my tribe. One time on blew up so I checked her health and it wasn't that low. Then the next one blew up. Her health was really low and only the circular part at the end was filled. But she hasn't died. What's happening?

The potions never kill anyone directly. As you've found, though, they can reduce the health bar dramatically. There's always the danger that another accident or sickness might kill her. I'd suggest making a couple more potions to restore at least some of the chief's health. Don't use the chief to make them this time, though!