Don't assume that because you're young it isn't a heart attack. I've heard of people dieing in their 20's due to heart attack.
I have experienced something similar, but I believe it was zocor, my cholesterol med. I felt like I couldn't breath because of an elephant on my chest. But I had pain in my lower left ribs not in my heart area. I ended up losing my dinner and started to feel better. But the pain was pretty bad. Before I lost dinner, I almost called 911. After word I didn't feel like I needed to anymore. It happened two other times, but to lesser degrees.
I mentioned this to my doctor and she said its possible I had a stomach bug, but even if it was my meds she wasn't concerned enough with 3 occurances that hadn't happened in a month to take me off the zocor when the results were working so well.
When I started HRT I felt nausious for a month as my body adjusted. So maybe I went through something similar with zocor.