Some people have a 'failure to thrive' on a vegetarian diet.http://www.beyondveg.com/
has some info on it (warning, site really isn't veg*n friendly -- its rather biased in
its own way -- which isn't to say its not useful!).
Basically, after switching to a vegetarian diet, some people develope health problems.
Due to a lack of research the cause is unknown -- it may be as simple as improper nutrition. It may be as complex as some sort of unknown deficiency or nutritional requirement present in a small portion of the population. Or perhaps its a coincidence. Nobody knows.
As a vegan, I watch my diet and don't have any major problems. Is it a cure-all? Nope. My triglycerides are
in the 500s (normal 'high-end' is 150). Do I have to pay attention to my diet? Somewhat -- I try to make sure I get a source of b12 daily, and watch my iron intake (cereal + soy milk helps alot).
Don't get the wrong impression -- the Standard American Diet (SAD) is quite nasty, resulting in plenty of preventable deaths (the number one and number two causes of death in the US can be the result of a bad diet). People need
to pay attention to what they eat - omni, vegetarian, or vegan.
We are all walking around with brains that are hardwired to believe that fats and calories are a scarcity, and that plant foods are plentiful. Yet, in the modern, western country, just the opposite is true -- we can easily gorge ourselves on fats and empty calories, and neglect plant foods.
Btw, none of us are medical professionals. Even though I'm guessing that there's a lot of us who know more about veg*n nutrition off the top of our heads then the average medical professional, please don't consider our advice a substitute for medical attention -- find a proper nutritionist or doctor.
That being said, we need more information -- How old are you? What sort of allergies are you developing? What is your typical weekly diet? Did anything else change in your life -- weight, environment, etc.