I agree with everyone else here--I don't think the problems you're having would go away if you weren't vegan/vegetarian, something else is going on. A doctor or dietitian is called for--DO NOT substitute what you read online for the customized, well-informed help they can provide.
Why am I having these symptoms, how much should I be eating, more than 2000 calories or about that much? and if i do transition to veganism again, how will this affect how active I will become.
The most commonly used formula for determining your calorie needs is the Harris-Benedict. Here are the numbers for you, given your age and size. They indicate different lifestyles you could be living:
Sedentary - Little or no exercise and desk jobs: 1,823
Lightly Active - Light exercise or sports 1-3 days a week: 2,089
Moderately Active - Moderate exercise or sports 3-5 days a week: 2,354
Very Active Hard - exercise or sports 6-7 days a week: 2,620
Extremely Active - Hard daily exercise or sports and physical job: 2,886
You would probably be AT LEAST "moderately active." You are likely more than that, I bet. You could try carefully tracking calories, eating a range of foods, and eating around 2500 calories or more (every day--not just on your exercise days). Do it for awhile, monitor your body and mood, and see how you feel.
Also, I have a personal comment on this part of your post. When I became a vegan, it took me awhile to adjust to eating enough. The sheer amount of fiber fills you up--sometimes I really have to force myself to eat more to sustain an active life and good mood. For the first months, I lost weight, including muscle mass, and think I was aggravating my depression pretty significantly (although it can be hard to pinpoint a single cause). I read Jack Norris, a vegan RD, mention he'd had a similar problem and he just needed to eat a bit more. For me, by simply eating more, things turned around drastically and I greatly improved my strength and overall fitness, and most importantly, my mental state.
I've looked into this and I think part of the reason not eating enough seems to be common with vegans may be the fiber in our diet. Soluble fiber is not digested to the full extent as other carbs--there are estimates that it may provide, at max, 2.5 calories instead of 4, possibly significantly less. This is not reflected on current food labeling in the US (not sure elsewhere), but it may soon change. If you're eating a high fiber, vegan diet, that can add up--particularly if you're already under-eating.
So, as an idle, ill-informed suggestion, I think you could try eating more. Explore more foods, allow yourself more of your favorite foods. With the running, lifting, and yoga, plus being a teenager, 2000 calories is not enough. Not enough calories can aggravate a lot of other things--trust me, I've had the problem.
Also, going forward, I would recommend the book co-authored by Virginia Messina, a leading vegan nurse. It is called The Vegan Sourcebook.