I read several theories.
1. The first is the one in that comic.
2. The second is that she was pregnant, ad either the child got infected, or she did but the child didn't, she killed the child and the human part of her was horrified by it
3. A theory saying that all the witches are the same witch, and she can regenerate from anything, isaware of this, and thus her crying is from the pain of the number of times she has died and been resurrected.
4. Was at that time in the month when infected
5. Rape victim, either being raped by the infected, or raped before and then infected
6. She's got just enough human emotion left to know what she's become, which is why she runs away when she's incapped you, because she's horrified at what she's just done.
7. Because it's really, really, really fucking creepy!
Take your pick