Hi welcome to Health care magic forum.
Thanks for calling H.C.M.
It appears to be an allergic reaction
to the cloths wearing or new foods eating in recent times, or to soap or powder used for her in recent days.
Other possibility could be a fungal infection
. What ever the reason is there is a possibility of secondary bacterial infection over the existing lesion.
I advise you to consult a dermatologist
, for diagnosis and treatment.