Hi friend,
Welcome to Health Care Magic
The usual reason in such cases is the inability to drain - the opening may get blocked intermittently... At times, it is a
sinus... / Underlying fat, folds and moisture add on...
Check you blood sugar...
Keeping the area clean and dry / use of local antiseptics can help
If not improving, see the surgeon...
There does not appear to be any relation tp your previous illness...
Take care
Wishing speedy recovery
God bless
Good luck