Sourdough starter gone wild
There are a couple of (potential) issues here.  

First, sourdough starter doesn't involve store bought yeast, so I'm not sure I'd call what you have a sourdough starter-- I would call it more of a poolish or a biga.  What makes it a sourdough starter is the cultivation of wild yeasts (this takes time, usually on the order of one to two weeks), but if you were to take care of and feed what you currently have for long enough, I would imagine that it would become a proper starter, although I would expect that to take more like months for the store bought yeasts to die off and be replaced with wild ones.

Second, two teaspoons of yeast for two cups of flour and two cups of water sounds like an awful lot of yeast.  If I were baking something right away that called for that much flour I don't think I'd use that much. Related, thirdly, you don't mention anything about the size of your container, but I suspect its too small. Typically expect yeast related things to double in volume, so if your container couldn't hold at least that much it is too small.