"Just" anxiety isn't an apt description necessarily, but it might be. Anxiety has a colloquial sense, in that you get nervous about something and it bothers you. Anxiety as a disorder is chronic and gets in the way of your life on a regular basis and is something that can take over your life. It sounds to me like you had a panic attack. The question is why having chest tightness alone would trigger that much of a reaction. Is this something you experience often or is the first and only time? Do you have a phobia about heart problems? Did you start to "breathe like crazy" because you got scared about the chest or did it just happen spontaneously? Anxiety is a thinking disorder, and if you have it physiological symptoms can follow. But there are also physiological problems that aren't caused by anxiety and even anxiety sufferers get all the physiological problems everyone else gets. Now, it you're young, and I'm assuming you are given you are living in the same house as your sister, heart problems aren't likely. So again, it could be something and it could be anxiety, and if it is anxiety, which your aunt is right about, it does sound like that, you need to discern if this is a one and done bad moment or a chronic problem. If it's the latter, you want to nip it in the bud by seeing a therapist or something like that. If it's one and done, life brings bad moments sometimes. If you think it's physiological because you're having chest and breathing problems at times and not just this once, you need to see a doctor. Peace.
I would agree with your aunt. We can really do a number on ourselves over thinking about something. One time I sat for a good while thinking about how my name sounded when I said it slowly. Like, my name which I've heard all my life but it sounded so strange and I just sat there obsessing over it. :-o And focusing on our breathing is something I've heard before. So, it can be something you are hyper focusing on, then anxiety kicking in making you almost hyper ventilate thinking about it over and over. OR it can also be a symptom of anxiety itself. https://www.medicalnewstoday.com/articles/326831 That article is about shortness of breath that some people experience when anxious.
I am sure your aunt knew if you'd been sick or not but with the current global pandemic, I do have to ask that you've made sure you aren't sick, right? Shortness of breath is a symptom of covid-19 but most people also seem to have other symptoms as well.
It's an anxious time in general. And if we are already prone to bouts of feeling nervous and anxious, this exacerbation can happen. I'm sorry you are experiencing that. I wonder if you tried perhaps distracting yourself should this happen again if it would help? If you had a go to activity such as reading, going for a walk, calling a friend if it starts to ramp up again. You can also try some things like meditation. Now, I have always not been able to meditate. But I have found some apps that are really helpful. Headspace and Calm are two that I have personal experience with. Very calming. Headspace is free and Calm has a free version although I pay for mine which is enhanced. Not sure what the difference is.
Are you anxious? Has this issue with breathing happened again? Or just once?