She has seen FAR worse.
I can only speak for myself, but walking in on my 12 year old son having fun and dancing in his room would never be something that I would worry about or be offended by.
Indeed, my son is 12. Not much younger than you!
You need to learn to laugh at it. You know you would laugh if you walked in on someone else doing that. You gotta be able to laugh at yourself. If I fall down I will laugh.
I would laugh. I have an amazingly goofy sense of humor. Like yesterday I was eating a McChicken at work and I had mayo on the corner of my mouth, and a guy I work with jokingly said you got something on your face, and I was like don't worry, it is only mayo. As I happen to be gay it was extremely funny. Everyone laughed.
My mum would have probably joined in :)
..and took a picture and upload it online o.o