Are you using a automated rank checker? if so check about one a week. This just happened to me, except the opposite I added content, and my keyword shot up 450 spots overnight.
Did you add any alt tags, banner, text link this can produce this effect. Look at it this way sometimes it takes a while for say Google to digest the new content and will temporarily move you back.
Give it a couple of weeks to a day, for accurate results. I wouldn’t worry about it too much. If you are using a automated checker, also check manually. greek39