Step-Daughter Thought I Was Sleeping

My adult 25 year old step-daughter and I have had a normal relationship from the start 17 years ago. She is a very attractive and petite, and always speaks whats on her mind. Although she is very modest when it comes to sex and things she doesn't mind showing off her body with tight fitting clothes. This is why I couldn't believe this had happened. A short time ago, she spent about month back at home after she and her boyfriend broke up, one night after spending time out with her girl friends she came home and hung out on the couch with me complaining about her luck with men, jobs and school. I was trying to enjoy a movie half listening to her, she was sitting sideways on the couch facing me and I was slouched down with my feet out on the table in front of me. she finally ran out of gas and fell asleep and I did as well shortly after. When I woke up I found that she had slid down and had her head just above my crotch with her hand under her cheek, this normally wouldn't have been a big deal I would have just moved her and told her to go to bed, but I had a raging hard-on that was running along side and touching her face. I sat there horrified hoping she wouldn't wake up and have both of us suffering a lifetime memory and embarrassment of having her wake up with that sticking in her face. I tried to think of everything I could to get it to go down as fast as I could. As I sat there I couldn't help feeling aroused and my shaft started to throb and get so hard I thought it was going to split. That's when I felt a sensation of a finger drawing along the length of my shaft. Is this really happening? What is she doing? I wanted to tell her to stop but I didn't want to embarrass her if she thought I was sleepy and was only sneaking a curiosity feel. My heart was pounding so hard I could feel it in my ears. I did everything I could to keep my breathing in check. She continued to trace the outline of my dick as my mind was racing out of control. All I could think about was wanting to tell her to stop but if she thought I was sleeping. Then bam! without warning I exploded with the most intense orgasm I have ever had. I came with huge ropes of cum, each shot felt like it was tearing through the inside of my shaft, soaking the inside of the flannel night pants, it was almost painful because I didn't move at all and was hoping she though I was having a wet dream. Every nerve in my body tingled, my head was spinning and I was sweating, all I wanted to do is take a deep breath. Then to shock me some more she slid her finger slowly into the opening of my night pants and stuck her finger into my cum and was drawing her finger around my shrinking shaft. She laid there for another 15 minuets or so before slowly getting up as if not to wake me up and went to bed. I laid thinking how can I look at her the same way again and did she know if I was awake. When I went to bed a little while later as I passed by the bedroom I stopped and could hear her bed rustling and slow low moans, it was her turn I guess. There has been no mention about it since, I pretend like nothing had happened, but I do notice she talks and looks at me differently, not in a bad way.