There is also another possibility that we often forget because we are closed up down there...an abscess. It could be a sterile abscess or cyst or something else that is growing where it shouldn't.
It can cause a feeling of heaviness, like something is in there (in fact it is!) and wants to get out!
An ultrasound can often find it easier than any other study. I had 2 tumours 5 and 8 inches in the 'Cul de sac de Douglas', the area behind the rectum in the bottom curve of the spine (sacrum coxys (sp?)). This was 10yrs post pouch creation and by the time we found them (no one was looking down there!) they were huge and adhesed on the spine. My first hint was the feeling of 'having to go'. In my case they were inoperable and I had to go through a lot of intense Chemo to get rid of them. I was very lucky. But since I had a long history with displasia they were not surprised.
It is worth looking into to make sure that there are no unfriendly visitors growing down there.