Hi there. How quickly did you make the change? Sometimes if you make the move too quickly, you can upset that delicate balance. I have been vegetarian for awhile now, and I am slowly, very gradually making the transition to vegan. It could be you have made too many changes too quickly. That happened to me when I incorporated green smoothies into my diet. I was doing them every day and was getting gas, but once I went to every other day, and worked up from there, it was not a problem. Now, I probably drink a quart a day, with no problems.
One thing you might try is a food journal. I know, I know, it's a pain in the arse, but it may quickly lead you to your problem. Everyone is different, so reading generic data about problem foods that SHOULD apply to everyone, is a bit of a farse. You really need to find what is bothering you, not everyone else. This has worked for me so well, that I always recommend it to others, who are not quite sure what is causing their problems.
With the food journal, making changes gradually, and the hypnosis program, I am IBS-free, and give thanks every day for this.