Is it normal to catch your parent?

I am a nurse and often work the swing shift.. I have a child and my parents often watch him while I'm at work. Last week I got at my parents at about 1am and was exhausted so I decided to stay the night. Since it was hot my mom was on the couch with the fan on her, my son and dad were in my parents bed, and i went to the spare room, which is by the living room. Anyways at about 4am I heard what I thought was my son moaning and talking in his sleep. Well it turns out it was my mom masturbating, since I had the door open I could hear her, and I'm sure she thought everyone was sound asleep. She obviously doesn't know I heard and I'm trying to be an adult and be glad she has a healthy sex life, but I just can't. I'm completely traumatized and freaking out. I would've rather heard my parents having sex. Is it normal to be overly freaking out about this? I'm so grossed out and so disgusted I just want to cry. Normal to react this much?