Whats going on with my weight?

I dont understand this, could someone please explain. Yesterday I weighed 59.1kg (130.2lbs), I ate good and slaved at the gym and had over 600calories in deficit. How come today I now weigh 59.8kg (131.8lbs)?
I always get excited to weigh myself after I know I've had a really good day beforehand but this is just so disappointing. I've been doing this exact same routine of eating well and having loads of calories left due to exercise and I was steadily losing about 100g per day, and now its done a massive jump in the other direction. This is such a put off.