Not sure what your diet plan is, but a 5 pound drop in one week is really nothing to worry about. It is nearly impossible for your body to drop 5 pounds of actual mass in a week. Most likely, you dropped a few extra pounds of water-weight, due to either a decrease in sodium, or reverse osmosis due to a sudden increase in water intake. If it continues, then it might be worrisome, but most anomalous drops in weight will even out quickly as long as you stick to your plan.