Suggest Cause For Fluids In Lower Uterus

My 19 Daughter has suffered UTI's since she was 14 years of age. She has had an ultrasound done and the results have said everything was fine except there was fluid in her lower uterus. Her GP said he didnt know what it could be but not too worry as she was young and he will re scan in 6 months if needed. What caused fluid in your lower Uterus at that age?