Bmi of 13, ought I be worried?

Need to see doctor. I usually like to provide concrete answers to questions here, but your query requires some definitive testing. There could be many metabolic, infectious, nutritional, gastrointestinal, endocrine issues causing the problem with your weight. Your BMI is dangerously low. You need to initiate a local work-up with a doctor right away. Please.
Yes. Bmi of 13 is very low, nl bmi is 18-24 and when bmi is low could be sign of malnutrition or eating disorder. Get evaluated asap.