What Causes Black Bruise On The Upper Abdomen While Having Systemic Lupus Erythematosus?

I have noticed that I have a black/blue style bruise on my upper left abdomen, just below rib cage. I cannot recall any blunt trauma, although recently I was assisting my 80 yr old mother in wheelchair, with transfers on and off toilets etc when out for a dr appt. I myself, have limited strength as a 57 yr old with SLE, I sometimes get lesions, but have never before had bruising on abdomen. It is a bit tender, what if anything should imbe doing for it.