Hi MrB.
When we eat a high carb diet our body needs to store water to help in processing the carbs. When you go to a low carb diet our bodies release that stored water. Hence the large initial weight loss.
One lb of fat equals 3500 cals. If you have been counting calories in the past without keeping carbs low you may have damaged your metabolism so reducing the number of calories you burn each day. But expect if you fast (as in no food) for 24 hours do not expect to loose more than 0.5 lbs.
However, weight loss isn’t linear. and the scales are not a good method of measuring progress for a number of reasons.
We are advised to drink 2 ltrs of water a day, a ltr of water weighs 1 kilo. So depending on where we are at in our water in / out cycle our weight can fluctuate by up to 2 kilos over the course of a day. .
Then there is the weight of what you have in your digestive system. 800 calories of cheese weighs a lot less than 800 calories of celery.
If you keen your carb intake down, so your insulin levels remain low then other hormones can also function correctly. That includes growth hormone, which is responsible for bone density, and muscle growth. Improving both will increase weight, so you will have periods where you have inch loss but no change in the reading of the scales.
Finally there is the whoosh effect. – Scientists are not sure why this happens. but what happens is that there is no change in the reading on the scales for a few days or weeks and then suddenly you release a lot of water and see a weight drop as a result. (the presumption that fat cells are filled with water as they are emptied with fat and then collapse releasing the water has been proved to not be accurate, but they don’t know what the real reason is. .)
So all told, taking lots of measurements and getting comfortable looking in the mirror for changes is much more effective method of monitoring progress than the scales.