Hello Gabriel, and everyone else who has reported on this blog a sudden shortness of breath, and the sudden onset of difficulty taking a full breath. This happened to me a while ago. It started about two days after having had a COVID shot, so initially I thought it was due to that. However, I have never had Covid, and have had no other symptoms related to Covid or any other medical condition for which shortness of breath seems to be related. It is a terrible feeling, and I thought that I must be seriously ill, or dying. I am a very active person, biking 20 miles several times a week. On days I don't bike, I hike 3+ miles (I live in the mountains so I'm hiking and biking at an elevation over six thousand feet), so I pretty much knew it wasn't my heart causing breathlessness. My symptoms always improved if I was active, or distracted in some way (busy around the house or yard, mentally engaged in something task). My inability to take a full breath also eased when meditating, doing yoga, and practicing Qigong, but would return shortly after practice. I too tried taking B12, and magnesium, thinking it was some type of nutritional deficiency. That seemed to work for just a couple of days. I checked my symptoms against the full list of conditions and diseases listed on the Mayo Clinic site that may cause breathlessness. What I found is that I have no other full range of symptoms of diseases listed by Mayo Clinic, other than Anxiety. I don't feel particularly anxious, or stressed out, I have a pretty chill life now that I am retired. However, some time ago, I started following news feeds. Before long the algorithm has filled my space with news. At first I felt like I could "observe" all the news without succumbing to the stress of constantly seeing and hearing and being reminded about every state, national and world event. Having now determined that anxiety is the only thing I can contribute to my sudden, very uncomfortable feeling of not getting sufficient breath, I have started to notice the tightness in my shoulders, and the clenching of my jaw, which I was previously unaware of. I have since "checked out" of checking in on the news. I'm being more conscious of stressors causing me to tighten my jaw and tense my shoulders, and it feels like a miracle, because I am able to fully breathe again I am happy and very relieved to report that my shortness of breath has all but disappeared. When, and if I feel it coming on, I check my body for clenching and tension and consciously relax and take a breath, breathing into my abdomen (diaphragmatic breathing), and bringing my attention to the present moment and relaxing, releasing the tension. Anxiety can sneak up on us. I thought I was pretty aware of my body. This has been a big lesson for me.
I wish for you all the best, and I hope my post will help you in some way to also find relief.