Rule 1: BMI is total boll0cks, it is possible to be over fat and within BMI.
Rule 2: Weight is irrelevant, body composition is what is portant.
Bareing in mind the above I think you are, strange as it seems, over fat and under weight.
Take a top class Rugby team, on BMI they are all Obese, clearly not the case so pay attentiion to the fat percentage not the weight. You may see from my name that I play rugby and by BMI reckoning I am 4 stone over weight!
Fat is calculated by passing a small current through the body and measuring variance in resistance. Depends on how good the device is but modern ones are considered pretty accurate.