I also weigh first thing but I also weigh before going to bed to give me a heads-up on what the morning will bring. For example, at this very moment I have just weighed myself and I am below my weight this morning. That spells a ma-husive loss tomorrow.
But on those days I am up 2 or 3 pounds in the evening compared to my morning weigh in that signals a gain. I normally have a gain once (sometimes twice) every week.
It's just how my body works.