What Does Fluid In Cul-de-sac Indicate?

last year I had pain in my left side abdomen ,my OBD did an ultrasound but didn't find any thing but she said that she found a big veine in my ovary kind of varicose ,so she put me on amytriptiline

this year my pain are severe in the same left side my new OBG said that I have to see a gasroanterologiste ,after many exam by the gastroanterologiste he send me back to the OBG ,this Dr dosn't take my ansuranceany more ,so my primery Dr did an ultrasoud and find some fluid in my cul the sac ,he put me in antibiotique Ciprofloxacin for 7 days ,today is my 5th day but no result

please explane to me what is this fluid in my cul de sac

the cause and the treatment

thank you