For what reasons might the tip of my penis hurt when I stretch backwards after ab workouts?

My idea. I've noticed the same thing. The abs attach on the top of the pubic bone, and the dorsal nerve of the penis runs right under. The abs being very toned after being worked, and then stretched more by your leaning back could pull the soft tissues that surround the pubic bone to put some stretch on the dorsal nerve. That's the best i can do. I'd not be concerned.