A bit like what I have experienced as well. I never got the diagnosis confirmed with tests, but I think I had COVID19 in March. I was ill but not very ill for a couple of weeks. Then I got better, but still after I got this shortness of breath from exertions. I am slowly improving but still not perfect.
Its called Long covid, and it happens to some, seems somewhat unrelated to severity of original disease. I find that the more active I am without getting breathless the more I improve. Face masks and being locked down in your home, is shit. Get out and walk in the fresh air.