The first bit of weight most people lose is water so half a stone/ a stone is not un-common in the early stages if you have radically changed your lifestyle/diet.

I doubt you will continue losing it at that rate, if you do, the chances are you're not eating enough and losing muscle instead of fat which aint good.

Work out your basic metabolic rate(calories needed to go about your average day and keep a stable weight)

Using this formula; 10 x weight(kg) + 6.25 x height(cm)-5 x age(y)+5

Then work out a diet and training plan to suit.

Drop about 500calories from your BMR to begin with, see how you get on for 3/4weeks at that level, if its not working, drop it in blocks of 100-150 but as a rule of thumb you should never go below 1500calories per day.

If you have good willpower and stick to your diet 6.75 days of week you will do fine.