Your grandparents sound like a couple of annoying old codgers who hopefully are doing this from a position of love and caring and not old-fashioned "morals" about unwed young people spending time together.

You mentioned living with them. Did you let them know in advance you were planning on crashing with a friend since they lived closer to work, or did you just leave them in the dark? Given the context they overreacted, but if they lacked context then their reaction is appropriate if a bit overboard.