One aspect of Billy's character is the fact that he has incidents of "time shifting" recurrently. Â For example, he could see his whole life from beginning to end in one "sweep".
In the chapter when Billy is transported to a camp in Germany, he and the other prisoners are taken to a feast by some English officers who were captured earlier in the war. Â Billy gets a shot of morphine that sends him time-tripping again. Â His mind was very likely to feel this sensations when some sudden circumstances appear.