On my fourth Vipassana meditation retreat, Ajahn Tong Tradition (variant of Mahasi Sayadaw), technique asks you to note "Rising" (breath at belly), "Falling" (breath at belly), "Sitting" (sensations of your body), "touching" (mentally touching specific points in the body, in sequence).

On the fourth or fifth day, my breath was very fast and shallow, and then suddenly it slammed shut. I could force myself to breathe again, only for my breath to naturally snap shut again. I found it very scary and difficult and was wrestling with whether I was somehow doing the wrong thing, if I was somehow "choosing" not to breathe, rather than it happening naturally. There is a massive grey area in there around "will" and "choice" when it comes to the breath, watching the breath naturally "as it is" becomes kind of an odd game when it stops.

When this happened, I discussed in several 1on1s with the teacher who said, when this happens, to replace "rising, falling" with just "knowing", and continue with the regular traversal "knowing, sitting, touching". To begin with, I was really fighting the not breathing, shaking, breathing weirdly. Her first advice was to drop the fight, stop indulging in it, and just watch. She also said I could also place my hands on my belly which may assist in feeling if the breath is there or not (maybe to prove to myself that it was indeed not there). Next, I started trying to go with no breath, and bear with it as long as possible, trying to get through one full traversal of the points. The game for that day was "how many times can you get through all the points". It was extremely difficult, I also had very strange things start happening regarding space squashing in to 2 dimensions, and time slowing really far down so that "reaching" each of the different points became harder and slower. The next day I told the teacher this, she said "there is no game, you shouldn't try to get anywhere", instead to note "trying", "striving", "doubting" and "confusion" etc. I started doing this, but ultimately failed.

I am still quite confused if this is expected because you'd think if you go for long enough without breathing, you would either die, or the body would start breathing again on its own (which never happened, or maybe I stopped it happening somehow). I was also aware that when you're meditating, you're not moving, and you're barely even thinking, so perhaps a lot less oxygen is needed. It was still a very strange exercise.

Has anyone experienced anything like this? Is this expected or was I somehow doing it wrong?

I have experienced something very similar to what you're describing, not that I can say whether or not you are doing anything "wrong", but eventually I sort of trained myself to switch gears whenever this would happen due to the near-excruciating pain it could cause, particularly at the area of the root chakra. Instead I began focusing more on bodily sensations, lower down like in the heart and belly, which seemed to take the spotlight off the breath, so to speak, so that my attention could rest more easily and I would have an overall more pleasant meditative experience. Don't get me wrong, there is definitely a value in addressing painful sensations with a firm mind, breaking them up a bit, but if something you're doing is getting gross responses it's probably a better idea to take the foot off the pedal a bit. For the bodily sensations I try to focus on the constantly moving around quality with periodic reiterations of positive reinforcement from the head, such as "kindness", "calmness", "ease", etc., usually moving my attention very gradually down in the body (it does this on its own) as the meditation progresses. Funnily enough, often I will realize that I may or may not have been breathing for a certain stretch of time, definitely, as a talk by Thanissaro Bhikku once brought to my attention, your body can breathe in other ways besides through the lungs, but when your mind is on something else oddly it isn't really that disturbing. Nowadays I still use the breath, but less frequently than bodily sensations, whereas at the time that I was having this total breath-out quasi-nightmare experience, I was slavishly and dogmatically chained to the idea of using breath as the sole object of my meditations.

Best of luck to you, hope this helps in some way.