Blame it on the scale. My scale and I aren't talking right now.
I have the exact same problem. I started off about 150lb, lowest in this diet has been 138.0lb, and then this week I'm back up in the 140s - about 142-143 (for honestly, the 17th time in 3 months)! It drives you crazy, but then I measured myself when I first started, I was 36-29-39. I'm now 36-27.5-37.5 and I started wearing a size 7, now can fit into a size 6. So that's one way that really helps when the scale doesn't.
One of my managers at work is insane with workouts and diet because he's training right now for something. Anyway, he was telling a co-worker and I the other day that in the summer months when it gets hot, your body actually tends to retain more water, thus water weight because of the humidity and heat. A good way to flush it out is to keep yourself hydrated, even though it sounds to contradict, it works.
Just don't give up. Keep tabs on inches and pants size. Remember, in the end, it doesn't matter what weight you are, as long as you feel good.