I assume this guy is not your manager. If that is the case, then maybe it is time to either talk directly to this guy (that would be best) and tell him that your work assignments should come directly from your manager, not him OR talk to your manager and ask that he/she make your work assignments, not other staff members.

This guy sounds like a bully and will probably take advantage of you until you either blow him off or take it to management.

((((Little Jay)))) A good work environment can be such a source of joy but a bad one can wreck your psyche!