There are several reasons why you may have pain in your rectum or perineal area (tail-end) after surgery.
In the early days after your operation it is not unusual to experience pain. The early post-operative pain is due to the removal of the rectum but the medical and nursing staff will give you painkillers to help with this. The pain will improve in time but painkillers are often needed for the first 8 - 12 weeks after surgery, taken regularly, these will ensure that pain is kept to a minimum.
You may find that you will not be able to sit comfortably for long periods of time post-operatively, but getting up and walking around will relieve the pressure from your perineal area.