How do you explain SI to someone if you think they've found out?

My mom was trying to be nice by installing my new printer while I was at a concert all day with my brother. My brother asked me to switch what CD was playing during the drive there, and since he has an iPhone that he was using to play the music I saw a text message on the screen from my mom. The text said, "Don't say anything to [my name]....what is SI?"...

The concert kept me mostly distracted, I still had it on my mind, but wasn't visibly freaking out. I'm home now. No one's brought it up, but my mom doesn't just let things go. She also has looked like she's been crying. I know she's going to bring this up sometime soon. I honestly can't see how I can lie my way out of this one. There's a little part of me that maybe wants to be honest about everything. That thinks maybe this is a way for me to finally get help. But more than anything else, I just want this to go away. I want to pretend that I never saw that text, that she never saw whatever it was on my computer that she saw. I just want this to go away...