15.8 bmi is not a dangerously low weight especially not to the point u wouldnt be able to walk. that would be 13 bmi and under.

anyway avoid going to the doctor right now as they might freak out and put a feeding t00b in ur nose. look into nutrition supplements like ensure+ thats made for weight gain and people who are malnourished. ヽ(´ー｀)ノ