This is the page where Charlie first mentions the truck that Billy Black had offered to sell him for Bella's use. Notice how worried he is that the conversation will take a wrong turn.
When Bella asks the year of the vehicle, the following is Charlie's response.
"Well, Billy's done a lot of work on the engine - it's only a few years old, really."
He's very defensive of the truck, as if he expects Bella to disapprove.
When I was Bella's age my own father bought me an old car. It was a Dodge Aries, from the mid 80's. If I had asked as many questions about the vehicle as Bella did, in the way that she asked (as though reserving judgement until the details were disclosed - also known as looking a gift horse in the mouth), my father would have changed his mind about buying it for me, and probably would have never bought me another thing. And the lecture I would have been subjected to about being thankful for what I have...I don't even want to think about it.
Of course, on page six Bella does not yet know that Charlie has bought the vehicle for her, or even that he was considering buying it for her, but it's pretty clear that he is trying his best to help her. Bella's critical tone jumps off of the page.
I don't blame him for feeling awkward around Bella.