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As you mentioned your butt hurts when you poop and often you have bleeding,now you feel being pierced with knife it is highly suggestive of anal fissure and you should seek dr advise for local examination of anal area to see exactly what is going wrong either its alone anal fissure or there is some degree of hemorrhoids,for temporary pain control you can apply locally lignocane gel available as over the counter medication.
Please get consultation as soon as possible and try to keep your stool soft,use low protein and high fiber diet.
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