Am I overreacting for feeling hurt because of a message of 'advice' MIL sent me?

Final update: Okay so everyone here was right, MIL completely made everything up. So loves our dynamic and he said he's never been happier. I showed him the message and he immediately called MIL. I didn't have to ask him to handle it or ask him if he'd told her the things in MIL's message, he clearly had no idea what she was talking about. He called her and told her off, he demanded to know why she sent this vile message. He wanted her to admit she'd tried to break us up, she first claimed she didn't send any message but I took a screenshot, and she claimed those can easily be faked. Why would I do that? According to MIL, I did it to increase the rift between them. SO told her I had taken a screen recording so she told the truth and said she'd sent it but her mother's intuition inspired it. She said SO doesn't look as happy as he was when he was with his ex. MIL claims she wasn't trying to break us up but was trying to get me to be more like his ex because he was happier with her. The NSFW stuff was inspired by what the ex told MIL SO likes. Yeah. He kept telling her to admit why she sent the message, why she freaked out and called me 29 times etc. She refused. He told her they were done and hung up. It's raining and she's outside calling SO's name and repeatedly ringing the doorbell which is off now.

Thank you all for your advice, it seems we're going NC with MIL.