Why does γυναικα turn to γυναικων - I'm aware the article has changed but why?
It is explained in the "Genitive" tips and notes. Please read them and if you have another question, feel free to ask!
Womens', right? What i have learned from other words the apostrophe should come after the letter "S" not right before it
That's not the rule.
If a plural ends in an -s, then the apostrophe comes after that; with a singular word, or a plural that does not end in -s, then the possessive is shown with an 's.
"women" is a plural that does not end in -s, so the possessive is "women's".
There is no word "womens", and so womens' does not make sense.