Sounds like you aren't eating enough. The types of plant foods you mention are indeed very healthy and excellent for the body, but they are not as calorific as meat, and they fill you up fairly quickly due to the high fiber. Meat also sits in the digestive tract longer, while plant foods move through faster. So you need to either eat more of them, or add more calorific foods like peanut butter, nuts, seeds, whole grains, or fresh/dried fruits.
As silva said, protein is nothing more than a group of amino acids, all of which can be found in plant foods, though only a few plant foods such as tofu and quinoa contain all of the amino acids in one food.
Three weeks is not really enough time for a vitamin or mineral deficiency to develop in the rare event one does, but you could have an intolerance or allergy to a newly introduced food, or your body might not be used to the added fiber and bulk. Beans are a difficult food for people to digest when they first start eating them. It is one of those foods you want to introduce more slowly if you aren't used to them. Over time you body becomes well adjusted to plant based eating, but for some people it can take a while.
Personally, when I went vegan I found that I needed to increase calories because I dropped weight very quickly even though I felt full after meals. Adding in a serving or two of nuts or seeds or peanut butter each day helped me with stamina and to stop the weight loss. I also found whole grains like oats, brown rice, couscous, pastas, and whole wheat bread easier to digest and satiating. Grains and nuts/seeds stabilize blood sugar also, whereas fruits and vegetables are more likely to cause spikes and drops in blood sugar. Just a thought. Low blood sugar or a drop in blood sugar can cause nausea.
In the end, only kindness matters. - Jewel
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