I don't know if it is normal or not but you are not alone in feeling the way you do. It started for me when kids would get angry and jealous when I did something better than them. It was always hard for me to make friends to begin with so in order to keep them I just played dumb or pretended to be bad in sports. I would tell myself, especially in high school, play dumb. My reasons for doing it have changed as I got older. When I do it now it is sometimes to spare someones feelings. I remember I was taking a calculus class with a girl I knew. Anyway, the class was hard, we both struggled, her more than me. When the final grade came in she was so excited. I congratualted her and when asked all I said was "Thankfully I passed". I did better than she did but I didn't want to take away from her achievement. I also do it because I don't like to be the center of attention.

I guess my advice is don't be afraid to be yourself. Always do your best, always. Most people will be happy for you and those who aren't aren't worth worrying about. Like the previous post said - dont be afraid to SHINE. Take it from someone who always hid.