This happened when I was 14. It was kind of embarrassing. I live with my mom and bro.
It was in the middle of the summer, on a Saturday so we were all home. I had just woken up in the morning, and it was about 9:00a.m. I decided to take a shower when I got up, so I went to the bathroom, and took off my clothes. We do not have locks on any of our doors because our house was built in the 40's. My grandmother grew up in this house. Anyway, we have the kind of shower/bathtub where you can slide the glass door shut and you can see through it.
I turned the water on and I was feeling horny so I decided I would masturbate. I laid on my back on the bathtub floor, and put the shower sprayer over my pussy. I was in there for a few minutes then all of a sudden my mom came into the bathroom to get the laundry. We usually do not have any problem with someone else going into the bathroom when we are in there, but this time I really wanted privacy. She said 'sorry' and left.
I was nervous and embarrassed but I kept going. I orgasmed in a few minutes, horny knowing that my mom saw me doing 'it.'
Later when we were both alone, she gave me little talk and said that it was okay to masturbate. I was happy actually because I really did not have to try and hide it anymore. It made me horny thinking that my mom knew.