Another possibility is that she was trying to make a point about body acceptance or a health issue. Maybe she had a mark or something on her buttocks that she wanted to show you in relation to a medical condition she had been dealing with, but she could have chosen a better way to do it.
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.
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.'
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.
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.
One possible reason is that she has a limited set of memories that are very important to her, so she keeps repeating them. Maybe those stories are the ones she cherishes the most from her past experiences.
Such a description involves violating normal family ethics. If it's a made - up story, it might be a sign of some very abnormal or inappropriate creative idea. In a healthy family, there should be proper boundaries between parents and children.
It could be that she was playing a game or fulfilling a bet with someone. For example, she might have lost a bet to a friend who dared her to dress you as a girl. Another possibility is that she was involved in some kind of theater or photo project at home and needed a 'girl' character, and you were the most convenient option around. Or perhaps she was just being creative and thought it would be a cute and funny thing to do.
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.
Perhaps it was a misunderstanding due to language barriers or unclear speech. The mother might have been trying to say something completely different. For example, she could have been talking about a little secret ('clue' misheard as 'clit') that she finally decided to share in a story - like way. It's important to clarify such things to avoid misconstruing the situation.
This statement is not suitable for normal discussion. If it's some sort of made - up or misconstrued situation, it goes against basic moral and ethical norms. A healthy family relationship involves love, respect, and appropriate boundaries, not sharing such private and inappropriate things.
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.