Thank you for your query.
Both flu and the medicine Tamiflu
can make you extremely weak which your daughter is experiencing. I suggest let her have adequate rest and a good nutritious diet and she should soon be on the road to recovery. Do finish the course of antibiotic which has been started. Flu can take some time, even 10-14 days, before the weakness disappears.
I hope I have answered your query to your satisfaction.
Wishing your daughter an early recovery.
Rajiv K Khandelwal