Happened to be turned sideways in my seat wearing a seatbelt while reaching for something in the back seat when the vehicle unexpectedly made a (very) hard stop. i now have a persistent pain in my low backl4-s1 area. possibly caused from this event?

It is quite possible, but a more detailed history would help make a fully informed opinion on causation. Although seatbelts save lives, they do affect the lower back by partially restricted certain movements. Furthermore, when the spine is twisted as you describe, there is susceptibility to the twisted segments.

Logical conclusion on your part, and likely the trigger to your symptoms. But the true pain generator might be sacroiliac joints, disc issues due to tearing of annulus fibrosis, or facet arthropathy. See your doctor, get appropriate studies, consider treating with a physical therapist.