What caused Bud to become so frightened that he locked Mr. Lewis out of his own car and tried to ride off?

In Chapter 10, Lefty Lewis pulls his car over after he sees Bud walking on the side of the road. Lefty gives Bud a drink of red pop and asks Bud where he lives. Bud lies and tells Mr. Lewis that he lives in Grand Rapids. Lefty tells Bud that he is also from Grand Rapids and walks Bud over to the passenger side of the car after Bud grabs his suitcase. Mr. Lewis moves a box that is sitting in the front seat so Bud can sit down. Bud notices that the box says, "URGENT: CONTAINS HUMAN BLOOD" (Curtis 106). Bud's wild imagination takes over, and he immediately believes that Lefty Lewis is a vampire. When Lefty lets go of Bud's arm, Bud slams and locks both doors so Lefty cannot get in. Bud attempts to drive away but is too short to reach the gas pedal and does not know how to operate a vehicle. Bud clumsily yanks down on the gear lever and the car takes off. Bud doesn't make it far before the car stops and turns off. Mr. Lewis then convinces Bud that he is not a vampire and Bud unlocks the door and rides back to Flint with him.