Hello Amy, 

Thanks for reaching out! It is difficult to say exactly what is wrong with your begonia, but it could be from overwatering. They are quite temperamental about watering, and overwatering may cause them to drop leaves. Cycles of overwatering and under watering can also cause leaf drop. 

Here is a helpful UMN Extension article with more information about begonias: https://extension.umn.edu/flowers/begonia

You also want to make sure the begonia is in a large enough pot and that it was potted in fresh potting soil. 

I hope this helps!