I Am Currently Due For My Periods. My Bmi Is

I am currently due for my periods. My bmi is 14.1 so for 18years haven't been able to conceive another child. Today I have had when ive gone to the toilet to urinate and wipe its been a maroon brown texture but not thick, no clots as ive had them before. Ive got lower left back pain. 6days ago I had extremely bad pain during sex & it hurt still afterwards for a short period of time.