How did i lose weight!? help please?

this happens to me alot and im very perplexed. i havent been to the gym in about a week, i have been laying on my couch all day/night watching tv, eating candy and other not so healthy foods. the only physical activity i have had is standing 4 hours for two days at work.(im a bagger at a grocery store and i push baskets and stuff but thats little work). so today i went to the gym and i stepped on the scale and somehow i have lost 4 pounds. how does this work!? i don't think i lost muscle because it doesn't go away after in a week. im 17 year old male and i (now) weight 160 lb.