Is this called feeling of apprehension?

i went zoo in 2008 with my nephew who was only one year old.i held him and i was standing on a brigde.beneath the brigde there were crocodiles..after about a year,i was thinking about a trip,an imagination suddenly came into my mind,i thought if my nephew fell from my hand there then what will happen,this thought scared me for a moment....what is this?why i thought like that?