fluid detected in Pouch of Douglas?

As a teenager I use to have a 31 day cycle period (heavily involved sports) and would often have some cramping/nausia during heaviest day of period. I went on the pill and never had any further difficulties, besides some slight cramping. Came off pill 6 months age. My period is now on a 27day cycle with very slight bleeding on the first day (really just blood when I wipe) then very light flow on the second, regular/heavy flow on the third followed by two days of light flow and occasional spotting on the last day. On my heaviest day I have moderate cramping, no nausia. My husband and I are wanting to try for a baby. So I went for my PAP smear and blood work. All came back normal, including cervical swap. However doctor commented that I did have some cervical erosion. I have only had one sexual partner. Went for transvaginal ultrasound (day 23 of cycle). Ovaries, uterus etc normal, but 71ml of fluid detected in Pouch of Douglas. What could this mean?