My 2 yr old son suddenly had a mood change. now very irritable; extra picky with food, sometimes no appetite; very clingy. what does this mean?

A few possibilities: First thing to do is get him checked by his doctor to make sure he doesn't have a physical illness- an ear infection, virus, or other illness can cause children to change their behavior drastically. If that is normal, check if there is any added stress- new changes at daycare, new household changes, etc. Children respond to different stimuli with fussiness and clinginess at times.