What Causes Black Scar Inside The Mouth?

Dear Dr,

Have a nice day..!

2-3 mnths back in my mouth a burnt like black scar occured..nw its spreading to next side f the mouth....v consult a internal med dr...he advisd me to gargle with mouth wash & to apply fungal cream...after applying it yesterday..nw i cant open my mouth bcoz pain...swelling....wen time f consultation he advised to chk sugar lvl & thyroid ... after tat he prescribed 25gm thyroid tab sugar 110 so control myself...after tat he told if its nt going within 1 wek v can go for an biopsy...so plz advs......

27yrs, female