Sudden 5.5lb weightloss?

So after stepping on the scales this morning I noticed that I had went from 175lbs yesterday to 171.5lbs today. 175lbs was my starting weight and I only started my weight loss journey 5 days ago. I am one of those people who weigh in everyday (at the same time, wearing the same clothes etcetc) so I can't really understand how I dropped so much weight literally overnight. I weighed myself again just now to double check, and I'm still 171.5lbs and I didn't eat too long ago either.

I'm reluctant to believe the scales at the moment so I want to hear your opinions before I recognise it as an achievement. I have been drinking plenty of water over the last five days, but then again it's all I ever really drink, I've stuck to my calorie limits and I've ran 3 times over the past week after starting the couch 2 5k program.

As it is the first few days of your diet it could just be water weight you've lost. In the first week you normally do lose quite a bit of weight. My first week of dieting I lost 5 lb and the week after xmas when I re-started a couple of years ago I lost 7 lb in a week (which a lot of that was obviously water weight) so I'd see it as a success. I wouldn't get too hung up on weighing yourself if you can. It's good seeing the number come down fast at first but when it starts to slow down and you see big fluctuations from day to day it can be a bit soul destroying. Keep up the good work