I am confused about this fiber thin, I have IBS and fiber in general does not work for me very well, makes me more bloated, but I still need it , may be in a very small amount.
I want to know, what is the difference between the fiber in veggies and grains

Vegetables carry more fiber, especially green-leaf vegetables. Grain does contain fiber, but not as much nutrients as to veggies.

Here's some veegies,grains and even fruit samples to show the difference in the amount of fiber:
Whole wheat pasta- 1 cup-- 6.3 grams
Oatmeal- 1 cup- 4 grams
Rye bread - 1 slice 1.9 grams

1 cup- peas 8.8 grams
Brussel spouts - 1 cup 6.4 grams
Artichoke- 1 medium- 6.5 grams
1 baked potato with skin- 4.4 grams
Corn - 1 cup- 4.2 grams

So the veggies have the best, if you can handle fruits some of the best are pears, figs, blueberries, apples, strawberries, oranges.
SO with your IBS, smaller amounts maybe on a regular basis, will give you close to what you need, then try builing the amounts up, a little at a time, if you can tolerate this. On average, men need 30-38 gr a day and women should try for 21-25 grams of fiber a day.