You're 70, and got back to cycling after a ten year layoff. Are you riding the same setup you had back in the day? If so, I suspect that you've lost (maybe temporarily) a bit of flexibility in your lower back and are struggling to manage the reach that used to be oK for you. So you'll tend to sit slightly more upright and since your hands can't come back, your backside slides forward.

That's just a guess. But if it's right you might try a shorter stem and see if that helps. A bike shop might help with loosening the existing stem for you.