Hello. What you are describing sounds very similar to a herpes simplex infection, which is possible to occur at the anus (which most people don't know). The Herpes blisters can cause quite a bit of discomfort to you, especially when even gentle pressure is applied (which would explain the sitting difficulty).
For a sure diagnosis, you will need to see your doctor who will do a swab and culture of the sores. He will likely perscribe you an antiviral medication to treat this. Because of the location, I would not suggest that you try any over the counter medications.
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