You didn't say your age, how old are you? You sound like what I went through. I have had so many tests done in a years period and everything always seems to be normal although I have felt really bad at times. If your hormones are fluctuating it could count for the anxiety. It is so hard to train our brain to realize nothing is really wrong just hormone fluctuations. That can cause problems with your bowels and digestive track. As far as the back when sitting, are you using good posture. what about the chair you are sitting in? I'm sure you don't have colon cancer. My obsession was I thought I was going to have a heart attack. I finally had to tell my self it was my anxieties talking. Hang in there and read the posts. Also go to the menopause boards and read.
We are here for you,
Jacq.