If you think it was just a really odd mistake on her part, you could try to laugh it off in a way that still makes it clear it's not okay. For instance, say 'Mom, that was a really weird moment. Let's not have any more of those.' And then change the subject. You want to set boundaries but also not make the situation overly tense.
You should probably tell her that it made you uncomfortable. You can be gentle but firm, like saying 'Mom, what you did made me feel a bit strange. Please don't do that again.'
You should react with respect. Listen attentively to her story and try to understand the significance she attaches to it. Smile and nod to show that you are engaged in what she is saying.
This is a very inappropriate and uncomfortable situation. You should immediately tell your mom that her behavior is not okay and that you feel uncomfortable. If it continues or makes you feel distressed, you might consider talking to another trusted adult, like a relative or a school counselor.
One should immediately stop the person from telling such an inappropriate story. It's not something that should be shared or spread.
Maybe it was an accident. She might not have realized you were in a position to see. For example, she could have been changing clothes quickly and not noticed you were there.
First, you should have an honest and calm conversation with her. Let her know how you feel about it, that it makes you uncomfortable and that it's not acceptable behavior. Try to understand why she did it, but firmly state your boundaries.
You could sit down and listen with excitement. After all, scary stories can be really thrilling, and it's a chance to bond with your mom. Just make sure you have a blanket or a stuffed animal nearby to hold if it gets too spooky.
It's a very awkward situation indeed. Acknowledge that you made a mistake in sharing such a story. You could try to explain to your mom that you didn't realize how inappropriate it was at the time. Then, make an effort to be more careful about what you share with her in the future. Also, try to do some nice things for her to ease the tension, like helping with the housework or making her a cup of tea. This can show that you are trying to make up for the discomfort you caused.
She might be shocked and a bit disappointed. But she also probably realizes that it was a mistake on your part. Just give her time to process it.
You could start a conversation with her about it. For example, you can ask her why she likes the story. It could lead to a great discussion about different relationships and acceptance. Maybe she's just curious about different ways of life and love. By talking, you can both share your thoughts and feelings.
Maybe she was trying to share a nostalgic memory. For example, she might have been reminiscing about a special occasion when she wore a particular lingerie piece, like her wedding day. It could also be a way to bond with you, showing a more personal side of herself.