Listen - weight fluctuates. It does that all the time, naturally and there's no rhyme or reason to it. It's not until you start paying (too much!) attention to the numbers that you realise this
Exercise can also make your muscles grab onto water while they recover. Also, if you're excersing, you can drop inches off your measurements without losing a pound...what do they say about lies, damned lies and statistics?
We all do it, but honestly - sneaky peaks between official weigh ins are (in my opinion) almost always a bad idea!
DO NOT be discouraged! I went away for a weekend a while ago and weighed myself on the Sunday to discover I'd put on three pounds
Those three pounds coloured the rest of my week, I thought about them a lot and they affected how I ate until weigh in day (Thursday). When I finally weighed in officially they'd gone. Despite a lovely weekend away, eating and drinking off plan I had stayed the same. If I had not weighed myself on the Sunday and just got straight back on plan I'd have been delighted with a STS - instead I was worrying about all sorts of needless things.
Carry on regardless!