What Causes Free Fluid Collection In Cul-de-sac?

hi, i am a mother of triplets.. i gave birth through the caecerian section... 2 weeks after birth of my babies i was takeen back to theatre because i had a perforated gut... am fine now fully healed. however, i have been experiencing some pain on the side of my lower abdomen. i went for ultra sound and the results are that i have free fluid collected in the cul-de-sac. what exactly does this mean? i am height 5.0 weight 63kgs and am 30 yrs old. please help