I`m no expert but I don`t really know what you mean by loin pain. Where is it exactly? Front, back, left, right side? I can`t think that loin pain would be a reaction to antibiotics. I suppose it could mean that her situation has progressed to a kidney infection. I`m not a fan of Trimethoprim (I got terrible thrush from that stuff even the GP told me that it wasn`t a side effect). Best for her to keep taking it though. Personally, I would listen to the GP rather than a pharmacist.