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Since there is chafing and thinning of the skin, there is some fluid coming out from the gut. Pelvic floor dysfunction, damage to the anal sphincter muscles or nerves of the anal sphincter muscles or the rectum, diarrhea, constipation, IBS, and loss of storage capacity in the rectum are a few causes. At your age it can also be a weak pelvic floor.
Some exercises to strengthen the pelvic floor, along with some treatment to control acidity, gas, constipation etc if you have will help. A multivitamin pill may also help, especially those with Vit B12. Please discuss this with your PCP and gynecologist as a confirmed diagnosis and management is not possible on net. Hope this helps. Take care!