PCOS makes sense, since I don't think she said what the mystery illness was. I bet she self-diagnosed since "she thought she couldn't get pregnant," not "the doctor told her she's infertile." If you aren't 100% sure (like total hysterectomy) that you can't get knocked up, guess what? There's a chance (however minute). And if your period is irregular, why not just take a goddamn piss test once in a while just to make sure?