"vortex shedding" means generation of vortices around bluff bodies due to flow sepration which are then shed/washed downstream because of the fluid flow. At certain flow regimes (particularly low Re) these vortices will generate fluctuating pressure (then forces) around the body which can cause it to oscillate.
Considering your Re number is relatively low and a sphere is not a streamlined shape, this is probably what's happening here; as was pointed out by Robert.