I experienced hyperstimulation during two of my births, both using self-hypnosis. I did not experience it as extreme pain. It was frustrating, annoying, and challenging, but no, it was not painful for me. However, the sensations made me fully aware that there was something unusual going on that required close watching. Since then, I have been told that the improved blood flow, extra endorphins and relaxation associated with self-hypnosis were what most likely kept my baby from going into distress, and are probably the only reason I did not have c-sections. I know that in the past, any suggestion that wasn't "true" brought me out of hypnosis, so I suspect that if the hyperstimulation had caused problems for me or the baby, the suggestions that I was safe and my baby was safe would have pulled me up, and alerted me to a problem. Since we were safe, I was aware of the unusual birthing pattern but all was going well, despite the complication, it makes sense that I was able to remain in hypnosis.
You may have had a similar experience. It is also possible that since your baby was already being watched closely and treated for fetal distress, there would have been no benefit for you from a decrease in your comfort level. You didn't need to be warned of anything going on, so your comfort level continued. Self-hypnosis does not "mask" sensations necessary to give us safety information, but you may not have needed a warning. For instance, one of my friends (who I'm hoping sees this and responds) had comfortable births using self-hypnosis, except for the one time she experienced a serious complication that threatened her baby's safety. She was able to use her skills to remain calm and clear headed so she could stay connected to what her baby was telling her to do and make good decisions, but she was no longer comfortable. In fact, I know some midwives who encourage VBAC moms to use Hypnobabies specifically because they know it gives the moms a better mind-body connection than they usually see, and they trust them to be more in touch with what is happening with their bodies. And they have concerns that a woman with an epidural is more likely to miss early signs her body may be giving her of a UR, while a mom using Hypnobabies has comfort without that risk.
One woman I know who was among the first moms I knew to learn Hypnobabies had problems staying in hypnosis a few hours into her birth. She kept coming up at the same point in the Birth Guide script, and it wasn't until they decided to head to the hospital that she found out that her BP was dangerously high, and her baby was showing signs of distress. Once they stabilized her and her baby, she was able to remain deeply in hypnosis and very relaxed for the rest of the birth. Her husband was the one that realized that the wording that brought her out of hypnosis was a suggestion that would not be true for someone who was experiencing dangerously high blood pressure, and that was why her mind would not let her accept that suggestion.