Celebrate the bonus! Most likely, it is water. If 2 lbs is your average, then a sudden drop is usually water. I would guess you cut sodium somewhere, or maybe you got some sleep you needed. You release hormones in your sleep that help you lose weight. That's one of the reasons why people with sleep apnea are usually overweight too.
If this keeps happening, then it might be something to look at, but if only 5 lbs, that is probably okay.