Before vaccines, your immune system was slow to respond. Viral loads could get very high before your symptoms (side effect of immune response) appeared, leading to lots of pre-symptomatic contagion. Rapid tests would often show positive at or just before symptom onset.
After vaccines or previous infection, your immune system ramps up much sooner, often causing symptoms to appear before the viral load makes you very contagious. This is similar to other infections for which we have prior immune experience.
It is now very normal to have symptoms, rapid-test negative, and not yet be contagious. I see a lot of people concluding that with symptoms and a negative test, it's probably not COVID. That would have been a reasonable conclusion a year ago, but not today.