Just had the head impulse test (HIT) / head thrust test done by PT. I was very scared by how fast she did it and how quickly my head moved. Given that the movement is meant to be fast, could this not cause or aggravate a concussion/neck injury?

The risk is too your neck but not very likely a risk of concussion.

Personally, I would never let anybody do such a test on me.
The instructions mention the need to not manipulate the vertebra. With this twist, the vertebra has to move quickly and the vertebral artery is contained in that area. A dislodging of plaques in the vertebral artery could cause a stroke similar to a beauty parlor stroke from extending the head into the shampoo bowl.

I prefer videonystagnography using the goggles and computer.

ABOUT HEAD IMPULSE TEST:
I had the head impulse test done on me last week. Besides causing a stroke (which did not happen), could it have caused neck injury?
I am very paranoid about this now and so annoyed that they did this test on me as my PCS symptoms flare up so often so the last thing I need is for a HEALTHCARE PROFESSIONAL to trigger my symptoms more. Is it not ridiculous that this test would be used for post-concussion patients then? I already deal with a lot of anxiety around making my injury worse so this is the last thing I needed.


ABOUT MANUAL THERAPY:
I also went to manual therapy for the first time last week. I went to two sessions. He worked on my entire neck going up into my skull and massaged pretty hard to get into my neck as there was a lot of tension and many trigger points. He also massaged the neck muscles near my jaw and in front of my neck at my throat. He did no manipulations or adjustments, just massaging.
After the 1st session I felt fine but immediately after the 2nd appointment I had much worse dizziness/unsteadiness and was feeling spaced out/brain fog. I also felt like my vision was a bit blurred and having trouble focusing my eyes on everything. It has been 5 days since the manual therapy appointment, and I am still feeling a lot worse dizziness and brain fog than before. I even have a light nausea which I did not have before this. These are my PCS symptoms I recognize but have definitely flared up a lot since the treatment. Could the manual therapy have messed up my neck?

Since I had the manual therapy and the head impulse test done in the same week, I am not sure what to attribute the new symptoms to. Given that the major symptoms started immediately after the 2nd manual therapy session, I think it is most likely that the manual therapy is causing the problem.

I am shocked that manual therapy could trigger such horrible symptoms. My symptoms this week have been worse than they have been in the past year. Have I completely messed up my neck or vestibular system now? I'm really scared because these symptoms are really uncomfortable and debilitating and I cannot believe I made myself feel worse from going to see a healthcare professional to try to treat my symptoms and make me feel better. Everything I read about manual therapy was really positive and saying that its supposed to help with PCS.

Did they tell you to drink plenty of water to flush out the toxins that are released during treatment?

There are good and not so good places.. speak up before they do anything sketchy.

Most places I've been to for PT & DC explain what they are going to do..before they do it..
If they don't tell you, I consider that a not good place or the person is not ....

For the manual therapy, the therapist did not explain in a lot of detail what he was going do but he did say he was going to work on my neck and that the muscles seemed quite tense. I don't think he did anything sketchy per se, it felt intense (but not painful or alarming) during the massage but I thought that was normal because the massage is supposed to release the muscles.

I do not think that was the problem as I also felt fine during the massage. If not I would have said so to him immediately. But right after the therapy was over I started to feel off but by then it was too late ...

It sounds like your neck therapist was too harsh. The neck needs gentle therapy that builds from session to session. The body needs time to respond to each gentle therapy.

To be honest, it did not feel like it was too harsh, it just felt like a deep massage. And my neck feels completely fine, no pain. My muscles were a bit sore the day after the massage, but that was more in my shoulders than in the neck even. So I really see nothing wrong with my neck. If anything shouldn't it have helped my symptoms?

If they were too harsh, could they have re-injured my neck? And how long will this flare up or new injury last?
I am quite concerned as I have developed a new symptom (a constant mild nausea/sick feeling) which I was not experiencing prior to the massage. What can I do moving forward to relieve this new symptom. I have tried some neck stretches but it does not help either.