No, of course not. It is your decision to shoot or not to shoot.
Just know, that the victim is now aware that there is something foul going on. He heared your bowstring, he saw a glint .... whatever. He is no longer suprised but "in combat" now. You may very well retreat (being stealthy as you are) and try again later, tomorrow or next week, but not in this fight.
In combat everything happens at once. It is just the needed abstraction of the combat round, that lets us think that the things happen in a particular order. Sometimes this seems to be important. As with the slow assassine. Nonetheless, it is your decision to not shoot if you are behind in initiative order. No one can take that away from you (at my table).