It's possible that your body is just catching up to past loss - mine does that a lot. I don't lose regularly 2lbs a week. I lose 4lbs here, 3lbs there, etc. It's just a quirk.
It's also possible that you could have lose some water weight. If you had a few very low sodium days, you might just have shed some water.
I'd say if it's a one-time thing, don't worry about it. If it continues, then go see a doctor.