Hi Kate, I'm sorry I wasn't able to respond to your question more quickly, and hope that by now your starter is rising well! If by "sponge" you mean a layer of frothy bubbles on top, this doesn't sound like a worrisome development, nor does a lack of rising for a few (or even several) days. Sourdough-starters-in-the-making are gradually weeding out unhelpful bacteria and working to set up a balanced ecosystem of wild yeast and lactic acid bacteria (LAB). The journey is not always straightforward, and weird smells and lulls often occur along the way. As long as you're not seeing any mold or pink/orange hues, and you didn't expose your starter to excessive heat (140ºF), it's worth being patient and letting the organisms do what they do. Patience almost always wins when it comes to sourdough starters. Good luck, and let us know how it goes! 

Barb

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