I believe it is normal, the thing has a mind of its own. A lot of times during high school, when I sleep in class (which was very often) and my upper body was bent over sleeping on the table, I'd catch a boner for some reason in that position. I'd wake up to the bell and going, "shoot, if I stand up to go to the next class...".
Maybe I said too much =/
Ya, I have the same problem, but I'm only 16. Sometimes on the bus, I will randomly get a boner and I'm afraid to stand up when the bus stops, I always stay sitting down a little longer than I need to and take a nice deep breath.