Trees

Yes and no.

Your THC is always degrading into other minor cannabinoids, but it happens very slowly at room temperature. Heating it up will speed that reaction up, though not by a ton until it gets pretty dang hot (this is why bud that's already been vaped tends to be very heavy in CBN). A week in a hot car probably didn't do wonders for your preroll, but depending on where it was (direct sunlight or shade, in the glove box or on the floor, etc), the amount of actual heat that the preroll was exposed to could vary quite a bit (my center console stays weirdly cool, for instance, but the glovebox gets very hot), so I can't say for sure if being left in the car degraded the THC.

That said, terpenes are quite volatile, and tend to influence the entourage effect of the weed, so you also could have lost enough terps to potentially dull some of the week's expected effects as well.