I have also experienced the extreme nausea and dizziness you described, along with a very real fear of never coming back. For me, the sensation was like a freefall and I was afraid I'd just keep falling through space. For you, it may have felt like a tunnel or expressed itself through your senses another way.
What I'd be looking at, if it happened to me, are the results. I would notice if the physical symptoms, such as the left-sided head pressure or the stiffness in the back, had eased or changed since the healing session.
I'd also be noticing any changes in dreamtime or waking consciousness since the experience that I would consider beneficial or positive?
Or conversely, have you had issues from the past resurface in a way that you can see will be ultimately healing for you?
I believe the fear of not coming back is more dangerous than just allowing ourselves to go when called. I think it's important to continually give over the fear to that within us which is larger than the fear, and to go.
To me, it has gotten less important to know where I'm going than to know I'm being accelerated by whatever journeys I am taking, whether in meditation, during a healing session, or in dreamtime.
As long as the healing facilitator you were working with was not pushing energy and energetically forcing you toward the journey, I would feel fine with that experience.
There are some healing facilitators who have not yet learned that, even though they can see a gridline of energy that could be traveled by the person they are working with - a gridline of acceleration, they interfere with free will if they attempt to force that journey.
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Spiritual Health Disclaimer: Spiritual journeying, whether in the context of a healing session or as the result of meditation, fasting, prayer, ceremony or dreamtime, is a very personal experience. The information contained in any health care / spiritual healing article or material on this site is not intended to define any person's spiritual journey but only suggestive of meaning I have found in my own times of deep journeying. Any action taken based on the contents found in this or any educational health information on Ask A Healer is at the sole discretion of the reader. Please consult with your chosen spiritual guides and counselors, and your own inner knowing, when assessing the risks involved in facing your fears within the framework of spiritual journeying.