Hello~Very generally speaking, fluid in the pelvis on an MRI scan is a very nonspecific finding. Anything that causes inflammation of the organs in the pelvis can be associated with fluid there. For example, endometriosis, in which tissue from the uterus escapes into the abdominal cavity, could cause this.
They probably thought it was a little unusual because ladies of our age (I am 60) usually don't have endometriosis.
If the doctor was not concerned however, and did not tell you to return for a follow-up exam in a year, then it more than likely nothing to worry about and is "normal" for you.