Has she gone back to eating properly, or is she now picking at food (this often happens after such a bout, especially in the elderly). If she isn't eating properly (and please understand I'm trying to suggest solutions here, not trying to make your life harder - I know you work very hard for your Mum anyway), can you try to tempt her with high-calorie, easy to eat food (full-fat ice-cream, full-fat milk desserts, etc) or try fortified milk-shakes.
Appetite drops off dramatically as people age anyway - an upset stomach will only make this worse.
I hope your Mum improves soon x