I don't know where you live, if you even live in the US, but that is a travesty that you weren't able to access quick testing. In the Bay Area, I get free testing through Project Baseline. My family has alternated getting tested once every 2 weeks, since our assumption is that if one of us gets COVID, then all of us gets COVID. Collectively we have been tested around 15 times,including children, and it's a breeze to get here.
But overall lack of testing capability is one of the stupidest things that happened over this past year. The US should have free testing available to everyone at least once a week. The capacity to test 100 million people in 2 days should have been built up, because testing is so vitally important to understanding what is going on. The fact we don't have that should be a crime because it has lead to so many deaths.
One thing to keep in mind is that a very bad flu went around in February/March. I know at least 10 people that thought they had COVID but didn't, because this flu was occurring at the same time. The other thing that many people have gotten mixed up with COVID is severe allergies. Try taking daily antihistamine to see if that clears up her coughing. Antihistamines need to be taken for weeks at a time in order to get good effectiveness, doing it one-off isn't nearly as effective. Her illness last March, be it COVID or not, may have made her more susceptible to alleriges or anything that irritates her lungs.