I don't think it's due to poor fiber intake.
Lifting heavy weights sometimes causes hemorrhoids. Also, sitting on the toilet too long can cause it too. The problem with most doctors advice is that they don't know what you did. They can only guess, and in my experience, most doctors tell you some sob story.
Best thing to do is simply stop lifting weights or pushing too hard for a little while. You can also buy a ring to sit on, if it's painful.