In your case, probably the history of trauma is not so relevant or amy be it is, as there is cololection deep to the sternocleidomastoid. It can be an infection or a resolving bleed as has been correctly described. You have improved on antiobiotics also indciates an infective process but it has agiann regressed. Yes it needsa spiration but before that i would want that we do a MRI cervical spine or neck especially to look for the underlkying spine as some times even TB spine can lead to cold abscess formation deep to the sternocleidomastoid.
Aspiration and its laboratory examination will also give us important clues to the diagnosis.