How and why did I get hemorrhoids?

I'm a healthy 26 year old male. I eat healthy, and consume plenty of fiber and water. I do not strain in the bathroom, and have never had this problem. Out of nowhere I get a hemorrhoid. The only thing I can think of is my multivitamin. I jumped on Opti-men for about a month, and then stopped as soon as I got the hemorrhoid. This vitamin is the only thing I changed in my diet/routine for the last year and more.