My pit bull is afraid of bugs?

When we first got my pit bull terrier, Kelsey, she would kill bugs with gusto and run around and shake them until they were mutilated. Now if she sees a bug she tucks her tail, freezes up, and lays on the ground trembling. As far as I know, she never got stung by a bee or bitten by any other kind of bug. What do you think makes her so utterly terrified of bugs now?