Im not sure why people are dismissing this - how many of us have ever decided "Right, tomorrow is THE day!" and then promptly gone and used ourselves as a human dustbin to clear out all the "bad food" before starting? It wouldnt be unreasonable in those circumstances to have a starting weight that was actually heavier than what we really weigh, and accordingly lose a substantial amount as that passes through the system.
Also, the fact that the OP appears to be on their first week of SW where we often (not always, but often) see a substantial loss the first week because the body reacts to the change in diet by ridding itself of any excess water it is holding on to and starting to lose may be a contributing factor.
OP, I hope for your sake that your scales are telling the truth. Dont be disappointed if come WI day they are over-exaggerating a bit, but I honestly wouldnt be surprised if they were telling the truth either.
I do agree with the others that there is a danger in hopping on and off the scales due to the various fluctuations that our bodies go through and that you should use your official WI as your guide but then conversely, there is NOTHING stopping our bodies having a wee "fluctuate" before the official WI either, which is where I suspect some of the unexplained gains that we see come from.
Look at your progress overall month by month, and if at the end of one month, you are lighter than the start of it, then you are doing things right