What is your age? In fertile age, this fluid can be due to pregnancy. In menopausal ladies it can be due to a serious condition. Anyhow, further investigations are needed including biopsy to diagnose your problem properly. Condition called adenomyosis can also cause fluid in the uterus. So, get advice from your doctor.
Fluid in the womb is called amniotic fluid. It is a clear yellowish liquid which surrounds fetus. For details, visit amniotic fluid.