First point here, I don't have asthma
When I was at school, I'd always get into a real mess when I tried to swim front crawl. My breathing would get out of synch, and I'd take paniccy lungfuls of air, and end up in in quite a lot of pain.
More recently, things have improved as I've calmed myself down, although I still struggle to swim more than a few lenghts of crawl at a time (although I can do breatsroke for miles)
What I have noticed though is on the days I take my sons swimming, I spend an hour or so just bobbing around in the water with them. Afterwards, my lungs are screwed:, it really hurts to breathe deeply. Yesterday I went for a road ride after swimming and it was plain nasty.
Quick google shows there's been some research into this and its not uncommon for the chlorinated water to trigger mild asthma-like symptons.
So, any advice how to avoid this, other than not swimming?