I wouldnt be booking her in for an op, this really doesn't sound like hormonal. If she was yowling due to wanting a mate, she would be escaping as soon as you let her out, and not coming back till she was 'satisfied', and would then be quiet. As you've only had her a month, it is still very early days in the settling in period. I have found a way to get my fosters to get through the night without crying, they just have access to all rooms apart from mine (purely cos George wants to go on the bed, and then Molly growls at him), but they do cry during the day when they are left, i either ignore them, or if they have done it so long I can't ignore them anymore, get told to shut up. Is there anyway you can shut her in a downstairs room with a nightlight and the radio playing? I would also be tempted to try feliway.