Background: The character in question is in her early 30s and is generally fit and healthy. She has three children, but has been celibate for several years before randomly having a one-night stand. She bore all of her children while she was in her teens (the youngest was born when she was 19). For plot reasons, I need her to not get pregnant from this one-night stand (not really a problem on its own), but to later find out that in fact, she is now unable to have children at all. She was previously a soldier but has been a non-combatant for some years now.
Setting is a fairly generic faux-medieval fantasy world. Medical "technology" consists of basic first aid, use of potions (complex to produce), magical healing of wounds and diseases, and some basic surgery. (Caesarian complications would be difficult to fit in to what I already have, however.)
I've googled "causes of female infertility", but most of what I've come up with seems to relate to innate conditions (i.e., the woman could never have been able to have children), or effects of STDs, which are unlikely for this character.
Help of any kind, from more useful search terms, through links to layman-comprehensible websites, to a straight-out answer, would be much appreciated.