It wasn't easy for you to suffer through what was happening, as you saw that the muscles on your body were shrinking to cause you to moan as you felt your own penis shrink while sitting there. You still had that attached to your body, but all of a sudden you felt cum spill out of it at a fast rate, making it smaller.
All that was left from your penis, once it was finished shrinking was a small opening as a uterus forms inside your body, along with a birth canal for any future child you would have. It was if something in the water must've had an effect on the moonlight while you were bathing in the hotsprings, and with the new genitals you have developed, you were becoming a woman as a result. Just then, you felt your own hips shrink to match that of a woman's hourglass figure.