Sudden loss skips the ending. No last conversation that knew it was last, no goodbye, no chance to arrange your face. One phone call and the world is simply other than it was. The mind cannot metabolize that speed, so it keeps re-running the tape, looking for the exit that wasn't there.
The last conversation you actually had was probably mundane. Something about dinner, or the car, or nothing at all. It can feel unbearable that the final words were so small. But listen: the smallness is the proof. You spoke to each other like people who had endless time, because that's what love does - it assumes tomorrow. Nobody talks in famous last words. The mundane goodbye means you were living, together, mid-sentence.
The what-ifs will run for a while: if the appointment had been earlier, if you'd called, if, if. What-ifs are the mind's way of preferring guilt to helplessness - guilt at least pretends you had control. You didn't. That's the horror of it, and also, eventually, the mercy: it wasn't yours to prevent.
Shock has its own timeline. If you feel nothing for weeks and then everything at once in a car park, that's not doing it wrong. That's the speed of the thing finally catching up.