I have both internal and external hemorrhoids. I, too, get the occasional blood sometimes more than others. I know it's due to pregnancies. I've had five pregnancies. Maybe once a year, I'll get a hemorrhoid flareup, too.
I, personally, think that is exactly what you are experiencing. Since they are internal, you don't know what might irritate one and cause the bleeding. Sometimes straining too much while you are going can do that.