I have a "lump" I get in my LRQ. It comes on a couple hours before I have a BM & disappears right before the BM. I think stool gets stuck in that spot & it becomes a lump, that can be felt since I am so thin as well. It finally breaks loose & I have my BM. I am not sure if this is what is happening to you or not.
Just so you know my history to understand, I have had a zillion tests as well. 2 colonoscopies, and EGD, MRI, CT, IVP, etc, etc. I have even had laproscopic surgery due to "masses" which were ovarian cysts. (that went away right before surgery so I had surgery for NOTHING.) I have had chronic pain & lost weight (50 lbs) since last year. It was thought I had ulcerative colitis or crohn's disease, but the last c-scope proved otherwise, so I have an IBS dx currently.
Live for today, for tomorrow you might just get hit by a semi.