Possible spoiled batch

I brewed my second batch yesterday and when I got ready to add my yeast I couldn't find it. Looked all over and I don't know what could have happened to it. Anyway I closed up my carboy and bought some new yeast today on the way home from work, but when I go to add it, the airlock was already bubbling and there is foam at the top of the carboy. I sanitized everything properly yesterday but I can't imagine what is making the bubbles except an infection. Any ideas?

 

How much did you drink while you brewed?!?, no seriously, it appears the yeast you can't find got pitched. Bubbling and krausen=yeast activity

If there were no live yeast in the wort there is nothing to cause an infection that quickly.