I've had a couple instances of similar discomfort since surgery. As was explained to me, during surgery some (connective) tissue between the prostate and rectum is scraped away/removed (from the rectum). This can cause a good deal of discomfort and take a few months to heal. My surgeon told me to call him if it continues. Otherwise my recovery from surgery has been going good.
I think part of it is my particular physiology, too. I've got a small butt and when I sit, I put a fair bit of pressure directly on that area...even when using a donut pillow. Also, I've increased my fiber intake which helps keep stool soft and keeps the bowels moving (reduces BM pressure).
Hope this helps. I'd recommend letting your surgeon know, anyhow.
Lower left groin hernia: mesh and large scar: surgery early 2006
Nov/Dec 07 and March 08 and now Dec 08: Severe perineal pain (between scrotum and rectum). Septra/Bactrim for 8 months (Nov 07-Jun 08) for diagnosed prostatitis.
PSA start of 2008: 5.3..... PSA June of 2008: 7.3
14 DRE all benign or nothing felt
TRUS Biopsy Nov 08: Got copy of pathology (see below). Prostate about 40 cm sq.
General Health: pretty good, 5' 10", 180 lbs, slim.
Bone scan Dec 08: Negative
Barium enema X-ray (March 09 due to several days of blood in stool)
MRI with endorectal coil (April 09 as part of a study)
3D advanced TRUS (April 09 as part of a study)
CT (April 09 as part of a study)
Biopsy Pathology: 5 of 8 cores positive, adenocarinoma in both lobes. 30%-65%. One core perineural invasion. 2 cores "foamy" and suspicious. All +ve cores, 3+3 GS 6.
Open RP surgery: May 5/09 Surgeon spoke to my wife and was very positive. Said both nerve bundles spared and not damaged. Bilateral lymph node dissection performed. Discharged 48 hours after surgery. Staples out, catheter out and pathology sheduled for May 21.
Physical State: Home office in basement. Up and down 10-20 times a day. Walking lots of circles around inside of house and back patio. Bit sore but coping very well so far.
Mental State: Pre-surgery anxiety gone. Now waiting for path report, cath removal and staples out.