I think that's normal, our at least it can be. Early on in my journey I had a 9 lb gain overnight. I've had a few 5-6 lb gains as well. It comes off pretty quickly, though, but not as fast as you gained it, unfortunately.
Oh my gosh I hate water weight gains, especially right before the TOM. I always put on like 4-5lbs in water weight and it'll stay like that for weeks and then just fall off. I really would rather it come off every week. Blargh!
I had a freak out this week because over the coarse of 4 days or so, I "gained" 5 lbs back. Part of it is the small gain from going off of Ideal Protein, but the rest turned out to be water.
The best thing to do is drink a tonne more water (sounds weird) to flush it all out as its retainiing either due to TOM, or sodium. Or whatever reason. Yesterday I managed to flush out 3lbs of water weight.