Having a BMI less than the 5th percentile puts you in the underweight category. About 4.2% of people in the U.S. ages 12 to 19 fall below the healthy weight range.
Just because you have a low BMI percentile doesn't mean you are unhealthy. BMI is only one piece of information. To know whether your BMI is unhealthy, you need to see a doctor.
Being underweight for a short while probably isn't cause for concern, but it is important to get checked out. Being underweight for a long time can cause health problems.