Si, you've lost far too much too quickly. Your body is starting to think your starving and its instinct is to preserve itself. Unfortunately this means storing fat, slowing metabolism and wasting muscle for an energy source. 2lbs a week is a reasonable loss. Start making small changes, get your 5 a day(minimum) drink pleanty of water, cut down on booze/currys/sweets, have a big breakfast, replenish carbs within 30 mins after excercise. Simple lifestyle changes are best rather than diets. The trick is to enjoy the excercise you do and have treats now and then, if you do pay the piper the next day.