Originally Posted by arg

Eating just fruits, vegetables, and nuts sounds like a tiring diet, and somewhat nutritionally unbalanced. I don't think many vegetarians could relate to that diet - it is vegetarian, but that's nothing like what most vegetarians eat. The fruits you're eating are probably giving you sugar highs and lows, leading to the lack of energy. If you want to stabilize that, eat some carbs that take your body longer to metabolize, to better regulate your energy level. Or eat very small portions very frequently.

Nuts give some people gas, so that's the likely cause, or legumes/beans if you're eating those as some of your vegetables. Most fruits, leaves, and roots would be unlikely to cause significant gas.

One thing you typically won't get from a vegetarian diet, even with grain-based carbs, is the long-lasting fullness you can get from fat-rich meats.

If you're not being careful about the nutrient balance of your diet, and you're going to be eating like this for more than a couple weeks, you should really be taking some supplements, particularly calcium and iron. You can get both minerals in various vegetables, but you could easily miss them if you're not targetting them specifically.