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Diarrhea can contribute to hemorrhoid formation because the bowel undergoes pressure strains due to the condition. Undue pressure on the veins that make up internal hemorrhoidal structures can worsen existing damage. Internal hemorrhoids are not visible, unless they proplapse (protrude) from the anus, either constantly or during a bowel movement. If this is the case, then you have a grade III or IV internal hemorrhoid. If the protrusion is on the anal verge or the area surrounding the anus, then its likely an external hemorrhoid. Reducing pressure from issues of diarrhea and constipation will help. Not straining during a bowel movement, not lifting heavy objects, not sitting for extended periods, all will help.
An external hemorrhoid is simply a vein located at the anal verge, the wall of which as weakened and protruded. Don't push on it, this will only increase pressure and worsen it. The body has to repair the vein wall and rebuild the integrity. This takes time, perhaps weeks, if all goes well. Use sitz baths and over-the-counter medications such as pads or creams to provide symptom relief in the interim. These won't solve the issue long term, but will give the tissue a better chance at self-healing.
Internal hemorrhoids are a different matter and require more intervention. Your best bet is to educate yourself about the condition, its causes and what options there are to treat it both short and long term. Don't ignore it though, hemorrhoids tend to become chronic in nature, lasting years or decades in some cases.