Please reassure me...Water Weight???

I am in need of reassurance! On March 17th, I met my 40 lb weight loss goals. Exciting, right? Short lived because I woke up the next morning and was 6 lbs heavier! Please tell me that it's water weight! I've since dropped 2 of those 6 lbs and now I'm 4 lbs heavier. So discouraging after working so hard to reach my weight loss goal!