I would sometimes get this feeling of nausea also. Just make sure that you give your body at least 45min to digest your last meal before working out, same goes for supplements. if you are having an extremely intense workout and get some adrenaline flowing, then once the physical activity's stop your adrenal glands release what is basically anti adrenaline(nor epinephrine) which may definitely make you shake and/or feel like absolute crap.