lower left swelling following bowel movement

Hi.  Wondering if anyone has any ideas... I have a mild swelling on the left side of my pelvis that becomes more pronounced after bowel movements.  (directly below and to the left of my navel) It also moves around in waves at that time, and it's accompanied by a palpable hard feeling directly above my navel.  I have had a CT, ultrasound, and MRI because my docs thought the issue was an ovarian cyst on the opposite side.  However, the cyst is resolving itself and there's no change in the swelling.  It is likely it is an unrelated GI problem, but no masses came up in the tests.  BMs are regular, but I have had a change to a vegan diet so consistency is different.  I also have tremendous amounts of gas and popping feelings in my pelvis on left side.  I should also mention that I'm very thin.  I wonder if it could be a gas pocket, as I've been told that air is not seen well on these tests.  As far as GI diseases/disorders go, I only know that I have had a borderline result on one of the celiac tests, the IGA I think.  My doctors are not very concerned, but they have not seen this strange sight after bowel movements.  Any ideas?

[live4fitness]

diverticulitis?

[goop]

it may be a hernia that becomes more pronounced after a bowel movement