Amniotic fluid refers to a yellowish fluid contained within the amnion membrane, specifically in the amniotic sac, which surrounds a developing fetus or embryo. This is not just a product of a reaction in the uterus, but it is naturally placed to give serious support to the developing fetus, specifically as it is acting as a shock absorber to the developing fetus.
A discharge, on the other hand, is usually used to refer to the liquid that comes out of women's vagina as part of their menstrual cycle. It is otherwise known as a vaginal discharge. A discharge can be caused by a lot of things and does not necessarily come because of infections or diseases, especially sexually transmitted ones. Amniotic fluid is formed in the uterus of a pregnant woman.
This means the presence of this material in the uterus of a woman cannot be caused by any other things except if a woman is pregnant.