If you're really worried then go to your GP. But the body is surprisingly resilient, also BMI is a poor indicator of actual weight to height/structure. e.g my BMI is about 16.4 I'm female, weigh about 7st and am 5'6". I AM underweight at the moment, I know this, I am putting weight on (or attempting to at least, just to clarify I don't have an eating disorder). However for my height, according to my BMI I should weight about 9st 4lbs which is HUGE! If I weighed 9st 4lbs I'd look and feel overweight. At 8st-8st 5lbs where my BMI is about 17-19 I am comfortable and I am also at a weight which my doctor agrees is sensible, but according to BMI's I'm still underweight. You need to factor in your build too.
Take BMI's with a pinch of salt, but please consult with your GP if you're really worried.