Either of these things are possible:

1. Because her MIL too might be evil with her and she is passing the same thing to you.
2. She is quite possessive about her son, and could not get his attention she was getting before, and his attention is all on you now.
3. She was already a bad woman who could not make friendly relationships with her MIL like behaving rudely & disrespectfully with her and now doing the same with you.
4. She is really insecure about her, about her son and about everyone. And has a perception that people do not like her already so she wanted to pose you in a bad light so that she appears as a better person than you and that it’s you who is misbehaving with her every time.
5. She does not have good terms with her husband, and thus could not see you happier with your husband. And in this case, she could not see all other women (like her friends, sisters, sister-in-laws) happy with their respective husbands.