Definitely echoing what Ian said. In your case, it sounds like it really is water.
Also, I'll add that women can easily gain even more than 2.6 lbs in a single day from water retention. I know personally that my weight can fluctuate pretty drastically - I'd give you numbers, but I don't weight in daily anymore so it's hard to know for certain. I can definitely say that I've gained 5 lbs in a day from water. (Over the course of a whole week I remember it being up 8 or 9 lbs once.

) But on the flip side, I'd also lose those 5 lbs practically overnight.
As long as you're absolutely certain that you're in a calorie deficit and sticking to your plan, you should have nothing to worry about. Be patient and it will catch up with you.