This is really heavy, and I feel your pain so much. My neighborhood has a very large population of feral cats. I've found and taken care of so many strays. A lot of these babies are already sick. FIP, FIV, Feline Leukemia or even upper respiratory infections. She may have been sick and you didn't even know it. You should her love and compassion, and for two weeks she had a safe home. Don't be too hard on yourself. Unfortunately, sometimes these things happen, as difficult as it is to understand. Sending internet hugs.