I'm so sorry for your loss. It sounds like she was weak all along. It may have been just general ill health and failure to thrive. There is so much we don't know about snails and why things go wrong with them that it's hard to say after the fact what was going on. It can be heart-wrenching to see this happen in spite of all we do to take good care of them, but many times there is nothing that can be done.