Retrograde menstruation.

Because the feliciano tubes aren’t closed on the ends, where they interact with the ovaries, blood from a period can flow backwards into your abdominal cavity.

Additionally, a small amount of sperm (if a woman is sexually active and say trying for a baby) can also go the full way and leak into the abdominal cavity.

So some women can have blood and semen free floating in their abdominal cavity. Between organs

Edit: autocorrect got me. I meant “fallopian tubes”