Any time your food intake, exercise and routine remain the same and you lose weight unexpectedly, at a rate of almost 5 lbs a month, you should have this checked out by your physician.
In the meantime, you may want to journal your food intake, calculate the calories and note the type of exercise, the intensity and the time involved.
There are calculators that can be used to determine the calorie intake needed to maintain your weight. You will need age, sex, height and weight ... one such calculator can be found here ... http://www.mayoclinic.com/health/calorie-calculator/NU00598
A separate calculator can be used to determine the calories you are expending in your activity level. You will need weight, type of exercise and duration.
Please note that losing 1 pound requires a net reduction of 3,500 calories. Exercising also plays a dramatic role in weight loss and the calories burned can be counted towards the 3,500 calorie deficit needed to lose a pound. So, in losing 23 pounds, you would have had to have had a calorie reduction of 80,500 calories.
Hope this helps .. please let me know if I can provide you with any additional information.