It's quite common for people to question their sexual orientation. This usually occurs in adolescence, but many adults have felt this way too - even after being in a relationship for years.
There are several articles online you may find interesting.
http://www.yoursexualorientation.info
There are also quizzes. Whether or not they are helpful is only for you to say. Here's one.
http://www.allthetests.com/quiz31/quiz/1410463821/What-is-your-sexuality
