Thank you for the input.Allergy
can be possible from any bite.
From history it seems to be an allergy.
Dermatitis or contact with an irritant is another possibility.
In bacterial infection such redness can be present but she should feel some pain when you touch the area.
The patch can be due to fungal infection too but fungal infection may not appear suddenly.
You can watch now how it progresses.
Allergy can subside by its own.
Sometimes we use steroid cream.
For small baby we do not like to use medicine, if it is not required.
If it does not show any improvement, it should be checked by doctor in person.
If you have any query, please feel free to ask & continue conversation by using reply button.
This will help me to explain further & satisfy you.
Wish her healthy life.