You started it with commercial instant yeast? Hmm interesting. I don't know how long the yeast will stay alive and able to reproduce itself.
If you want to start a sourdough culture with wild yeast that are naturally compatible with bread flour and never needs sugar (commercial baking yeast actually comes from the brewing industry) then you'd enjoy poking around this website:
http://www.sourdoughhome.com/starterprimer.html Everything you could possibly want to know about making and using your sourdough starter