American consumers are getting more nervous, and this morning’s data shows exactly why. The University of Michigan’s Consumer Sentiment Index fell to 48.1 in September, down from 51.7 in August — a four-month low. Consumers’ expectations for their own personal finances weakened by roughly 10% month over month. The reading came in slightly above the Street’s estimate of 47.5, but that’s cold comfort against a backdrop of rising grocery bills and gas prices squeezing household budgets nationwide.