Reasons why IBS develops are not clear. But it may result from or cause tightening, weakening, or in-coordination of the pelvic floor and abdominal wall muscles. The goal of treatment is to relieve symptoms. Stress reduction, dietary changes, fiber/water balance, and medication may be used in combination depending on the person as the conditions differ from one person to another.
Digestive health and regularity can influence sexual pain because the back side of the vaginal wall and the front of the lower bowel wall are adjacent to one another and the external openings of each are in such close proximity. Constipation puts pressure on the vaginal wall. Bowel movements that are too large or too hard can cause tears in or around the rectal opening. While loose bowels and diarrhea can cause an inflamed and sensitive rectal opening, and that area can certainly be further irritated by repetitive wiping.
Use resources available on the market to further study and understand IBS, like "IBS for Dummies".