Some background: I'm just turning thirty and work retail, on my feet and up and down stairs a hundred times or more in a day. I'm of a pretty slim build and have no car, so also walk everywhere.
I've never really considered myself "fit" but I didn't think I was unfit either, if that makes sense? But in recent years I've noticed I really struggle with heavy lifting at work - my chest will become unbearably tight, like my lungs are about to explode, and I can't seem to catch my breath. (This is after taking one 20kg box up a flight of stairs.)
It's a bit embarrassing but I've been putting it off as nothing until this week, when we were all sent home due to the coronavirus. I tried to do an online workout, just to keep myself active while stuck at home. It was a Joe Wicks one. For kids! It was five minutes long, with breaks of twenty seconds after doing forty seconds of one simple activity, eg star jumps.
I made it through the first forty seconds of marching on the spot okay, took twenty seconds rest, then tackled the star jumps. I couldn't go on after that. Pushed myself through the forty seconds but collapsed on the floor coughing and shaking after that. It took me fifteen minutes to recover. 
Is this normal? I know it's not the kind of exercise I normally do, but my reaction seems so extreme. Should I be worried about this? Or just keep at it and hope I'll get fitter in time?