We claim the ornamental design for a collapsible bag, as shown and described.
FIG. 1 is a top, front, right side perspective view of a collapsible bag in an expanded and opened position. The broken lines depicting hooks represent environmental matter and form no part of the claimed design.
FIG. 2 is a right side elevation view of a collapsible bag, the left side elevation view being a mirror image.
FIG. 3 is a front elevation view of a collapsible bag, the rear elevation view being a mirror image.
FIG. 4 is a top plan view of a collapsible bag.
FIG. 5 is a bottom plan view of a collapsible bag.
FIG. 6 is a top, front, right side perspective view of a collapsible bag in a collapsed and opened position; and,
FIG. 7 is a top, front, left side perspective view of a collapsible bag in an expanded and closed position.