What Causes Free Fluid In Douglas Pouch?

Hi, my mom is 58y old, 14 years ago she has done hysterectomy and doctors left one ovarian due to hormonal matter.She has been regularly doing the medical check and everything was fine. 3 months ago she has been doing gyn ultrasound and they found free fluid in douglas pouch 1,5cm. She has run the blood test including ca-125 and redults were negative. 3 months after that she went on an ultrasound again and fluid was again there now even increased in amound 3cm. We are very concerened what s going on. She will be doing colomoscopy this friday. She doesnt have symthopms that would indicate any other medical concern. Pls help with your advice. Thank you