I’m guessing stretching in the bath and jelqing in front of the heater are things you’ve done before or do regularly, do remember what you ate that day?
It was either a coincidence, or there was something different about that day that increased the chances for it to happen, and food can make a difference. If you were sitting in a certain chair for awhile on that day that you don’t usually sit in, maybe that could have made a difference.
I’ve seen a few long term injuries where the person said they were either jelqing in the bath, the shower, or applying heat while jelqing, so that’s why I asked. It sounds like you are staying near the heater too avoid getting too cold while jelqing, so maybe the way you are doing it, it’s not excessive heat but enough to avoid getting too cold.