I am not a Dr. nor do I play one on TV but I don't believe that riding causes hemorrhoids. It may irritate them however once they have flared. Hemorrhoids are caused by a number of reasons. Lack of Fiber, too much fatty foods, prolonged periods of sitting or standing, and last but certainly not least genetics.
Some people just get the ornery things no matter what they do. One thing that I have learned is the key and that is to stop focusing on them. It is ok to treat them if there is too much discomfort but don't let it rule your life or kick you off the bike.
Your doc should be the one to have a look and tell you for sure but I can tell you from experience that they are like kids. Once you have them you likely have them forever.
Just don't let them take control of you.