Hello and hope you are doing well.
It could most probably be tonsillitis and a throat infection. Throat pain, difficulty swallowing, fever, scratchiness in the throat, post nasal drip, malaise and loss of appetite are symptoms of tonsillitis. Also, white patches are common in many throat infections. The sore throat and tonsillitis which you had may have contributed to it. In response to the infection the body produces soft white debris which collects in the tiny indentations in the tonsils called crypts. Consult your primary care physician who will do blood tests and take a throat swab to determine the cause and prescribe appropriate medications.
Hope this helped and do keep us posted.
I've gurgled salt water too without any benefit.