Mom found a cum stain on the sheets!

I washed my old sheets and then put them away in the laundry room & grabbed another for my bed. Today, my mom used them and she called me into her room asking if I had sex with someone. I haven’t. It was a stain from when I played with myself (I’m 17 btw). I didn’t say anything and idk what to tell her, I just said I hadn’t had sex with anyone. I’m embarrassed 😳. How can I take it off?