Mally - is your goal weight really 8 stone and you are 5 foot 6? That's underweight! I'm the same height, and so am familiar with the weight ranges.
BMI Calculator
With reference to the weight showing, I personally at times seem to lose weight and it doesn't show, and then all of a sudden within a few days I have a 'big shrink' and my clothes suddenly all seem too big.
Please look and think carefully about your goal weight though. There are risks to your health by being underweight, my Mum spent a lot of her younger years being underweight and now suffers from osteoporosis which she has been told is a direct result. Her bones break so easily - last year all she did was stand up from the sofa in the wrong way and she broke 5 bones in her foot and was in a cast for more than 6 months as the bones also take a long time to heal.
You really need to weigh more than 8 stone at 5 foot 6 - please don't take this in the wrong way, I'm only saying it out of concern.