Brand new to Begonias, she hates me

This is my first ever begonia. In my defence, I very successfully grow many other plants, honest. I saw this in the reduced section and I thought ‘why not’. She started with all broken stems and two leaves, and in the first couple of weeks grew loads of new stems and leaves. I was very happy. I was almost smug. Then after her last watering a few weeks ago, she simply THREW all her leaves on the ground. What did I do? The smug has left my body. Please tell me what I’m doing wrong, otherwise she’s going in the bin to protect my ego.