How do I get my mom to stop asking me about what happened?

I went over to my guy friend's house Saturday morning, just because it was his 15th birthday (im 14) and I just wanted to ee him. We've been best friend's for 6 years, and this was the most embarrassing thing that has ever happened to us. I went over like at 10, and his mom told me to go upstairs and wake him up. I did go wake him up, I even jumped on the bed to try and wake him, but he wouldn't wake up. So I decided to pull his covers, and when I pulled it, I found out he was naked. I honestly did not expect that, and when I saw what I saw I just screamed, I think it sorta freaked me out. He looked at me, and I ran out of his room as fast as I could. His mom saw me, and she asked me what happened because she heard me scream. I told her I just needed to go home and get something (that was a total lie). Then she called my mom and asked her if I was ok, because she was worried about me and she asked my mom what happened. So my mom came to me asking me what happened, and I told her that I really don't want to talk about it. But now she keeps asking me, because his mom keeps asking my mom if she knows anything (I'm guessing he didn't tell her). I just really don't want to talk about it. How do I get my mom to stop being so nosy and leave it alone?