The lateral flow tests are a waste of time.
Three weeks ago I had Covid symptoms so over a two day period I took three lateral flow tests and all were negative. I wasn't convinced so I arranged a PCR test which came back as positive. I immediately took another lateral flow tests just to see and this still showed as negative.
I mentioned it some time ago, but there is this misunderstanding regarding lateral flow tests. It was never designed to tell someone if they are infected, but to determine if you are infectious, that's why people are supposed to do one on the day of an event (ideally) if they haven't been vaccinated. A PCR test is entirely different, and many times more sensitive and can work out if you are infected (although even this isn't 100% fool-proof). In your case, you could well have had covid symptoms, but your viral load may not have been enough - and therefore not infectious - to be picked up by the lateral flow test.