is it a fissure ?

hi All,

just wanted to ask you a question about a possible fistula/fissure.
Basically I feel like I have a small ball between my anus and my sitting bone. It feel deep in the buttock. When it first developed, I lost some fresh blood in my stool but it cleared out after 2 days. I also developed external roids which are no longer present.
I have an office job and I sit 8 hours a day. When I am sitting, I feel some pain and some kind of pressure ( ball feeling ) in that area. If I am on holiday and I do not sit for a few days, this feeling goes away and it clears away. Sometimes it feels like a stabbing pain and it is localised only in that area.
I went to a specialist 2 months ago. He looked inside my recturm and could not find anything wrong. He also did an unltrasound of my sphincter and an MRI of my pelvic floor. He could not find anything wrong. So my pain is still unexplained.
Do you think it is possible that the MRI missed a small fistula deep in my buttock ? What about people that have a fistula, do they feel like they are sitting on a golf ball or something. Do they feel a stabbing pain when sitting ?

The pain is deep in my buttock. And neither I nor the specialist could feel anything abnormal when pressing on it. And I think an MRI would have picked up an abcess. It all started a year ago when my pudendal nerve got inflamed because of a problem in my lower back. Now the nerve is OK but I still have this problem which does not seem to be the nerve because it is localised in that area only.