Unknown anal chronic pain

I have been suffering from a kind of anal chronic pain for 2 years , I am Chinese, 18 , male , my English isn’t very well , please read through my description below , I beg you .

One day , I suddenly get a diarrhea after that I feel like I get IBS, irregular diarrhea and Lactose intolerance , I was a healthy man , and I had a strong Intestine , I could drink milk without any discomfort.Since that day my life changed a lot.But what bothers me most is the anal chronic pain ,I will explain that more detailedly below.The pain must happen after defecation, not in the defecation , actually , it isn’t pain, it’s a strange feeling , I feel like I haven’t finish the defecation ，there is poop left in my anal , but I can’t poop more , and 10 minutes later I feel there is fart block my anal ,only putting it out can relieve my discomfort but soon it will come back . It will last 4-6hours until next defecation .It is worth noting that the top half of my stool is normal（like banana）, but the bottom half is soft and formless , it’s in the shape of cone ,every time. So to figure out what is the disease. I did a lot of medical examinations, including gastroscope, colonoscopy , Hydrogen and Methane-Based Breath Testing , anoscope , lumbosacral portion MRI , rectum ultrasound examination , but the all results are normal , only anoscope shows anal tube thickening .The examinations are done in Chinese top 5 hospital , so I think the results are credible.So I turn to foreign medical institution.If anyone knows anything about the disease please tell me, I’ll be very grateful.Thanks for reading.

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