Hi there,
I am pretty sure I have IBS to go with my general anxiety- I have never been properly tested for it, but was born with a spastic colon - aka IBS. I am currently taking prozac for my anxiety, but am finding that it (and other SSRIs like effexor) is helping with my IBS and digestive issues.
The main thing to indicate it might be Crohns or Ulcerative colitis instead is if there is blood in your stool. Also, IBD is usually only diagnosed in children, teens and young adults, so if you are over 25 it is highly unlikely. (I know a bit about this as my sister has UC)
As for ceiliacs disease, do you find eating gluten/wheat makes your symptoms worse ? Have you ruled out lactose intolerance ?
I would think it very unlikely you have parasites unless you have been holidaying somewhere exotic.
All your symptoms sound like IBS to me - I get gas pain, tummy cramps, diarrhea, no appetite. The best thing is to relax with a hot water bottle if you can, try and figure out which foods you can tolerate and take immodium, pepto bismol or laxatives depending on your symptoms. Aloe vera is also soothing, I find. I also have Levsin, which is an anti-spasmodic drug to take if i need to.
And I know that IBS is very real and thoroughly miserable - I am sure it partly causes my anxiety and even sometimes makes me feel depressed. Who wouldnt be with an upset tummy a lot of the time ?
Also with IBS, unfortunately the more you worry about it, the worse it tends to get. So try to relax (I know only too well this is easier said than done!)
best of luck