There is no such thing as loosing fluid if you are drinking plenty of water you body will keep the fluid it needs and remove what it doesn't (I owned 5 gyms)

Your not eating as much as you might have been before or you are making smarter choices, that is all. It can be a shock to the body so it loose;s weight fast. I have been doing WW for 8 weeks and lost 7kg so far. I am so happy with it. Good job exercising as well, and dont forget muscle doesnt weight more then fat.. 5kg is 5kg.. regardless.. Its your body making you slimmer and more tonned and so your body will change shape even if you are not loosing you are changing fat into muscle, becoming a leaner simmer person!

Good luck and well done!