referring to "what is the substance or material that is supposed to run down or off her leg if she is actually guilty"
The Chizkuni, a 13th century commentator looked at the language of "your thigh will sag and your belly distend" and understood it to mean
“you are no longer capable of engaging in carnal relations as the waters will damage the organs designed for this. Your womb will be ruined.”
Oh, wow... Okay, thanks.