Sounds like possible proctalgia fugax. Its basically pain in the rectum for unknown reason. Constipation can make it worse. I suffer from this occasionally and yes you are right, it does feel like knives jabbing. My doc says its the muscles contracting and spasming so I take an antispasmodic when this happens. You may want to ask your doc if there is something he can give you for the pain.
Other possible causes are hemorrhoids or anal fissures.
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Dx: IBS 1989, Diverticulosis 2004, Idiopathic Acute Colitis 2006, UCTD 2007
Meds: Plaquenil 400mg, Chlorzoxazone 500mg, Lyrica 50mg, Protonix 40mg, Naproxen 1000mg, Klonopin PRN, Miralax 17g, Supplements