Gaining weight overnight?

So I'm 23 weeks today, and this past week I stepped on the scale and seemingly gained 4lbs virtually overnight. I was obviously shocked, as I've been really careful not to eat junk food/too much food (I was a little overweight in the beginning). I've been mostly gaining weight slowly, which was the plan, and this was a complete (and unwelcome) surprise. My blood pressure is fine (if anything it's lower than normal), and I haven't gained anything in the last couple of days so I guess it's fine. But has anyone else experienced this before??