I'm brewing an English pub bitters I bought in a kit and everything went fine until I had an overflow of the airlock. It foamed up and shot all on the walls... Anyways, after cleaning it all up it was fine. There was a crusty residue in the carboy to where the foam went up but other than that it was fine. The smell when it was bubbling was a sweet, very beer smell... It smelled great! After a day of this the smell turned to a pretty bad sour smell... Should I worry?