I agree that there is an adjustment period when a person first becomes a vegan. That period almost never lasts more that a week. My guess is that you are either eating a poor vegan diet or there is something new that you are eating that you are alergic to. I would start with seeing if there is anything that you are eating now that you did not eat before. If so than try eliminating it to see if that helps.
As far as gas, some vegan foods will produce gas. Of course beans will do that. Some people have trouble with tofu. Just check what is new with your diet today as compaired with your old diet. That should help you find the problem.
A good vegan diet should not cause gas or diarrhea. Do not give up helping the animals, our environment, and your health just because you are having a problem that can be corrected.