loged in user name is inserted as iusr_servername in asp.net web form

hi,
 am having a ui which inserts a logedin user's name into the table.
till yesterday it was showing the current loged in user only.but today onwards, it was showing IUSR_servername
as the  logedin user name.
can anyone help me, why this in correct data is inserted into the db. its happned without any changes. i didint make any changes in the production code./ didin redeploy also.
is it the  iis user name??? prefix with IUSR_SERVRNAME?
 
 
 
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