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It is probably menstruation (which is mentioned often) or something I am not sure has been mentioned by that point. (view spoiler)[It could be her after she takes the herbs to make her infertile. I don't believe Cashore ever explicitly stated that she took the medicine. I believe it was hinted at which you may just be having trouble picking up on. (hide spoiler)]. Sorry, I don't have the book on me but it is most likely one of the two.

Veronica's right, but not about menstruation: (view spoiler)[ She DID take the herbs, and that's what made her sick. It was pretty confusing until she confessed to one of her friends that she had taken them. And that was AFTER the symptoms went away. (hide spoiler)]

It's implying she took the herbs, and she didn't want to break down and cry over the fact she won't be able to have children, because she did want them. She just couldn't risk bringing another monster like her father into the world, so she made the ultimate sacrifice. That was why Cashore created Hannah, to give Fire the child she couldn't have herself, to give Fire happiness in that matter.