NovoRapid reaction

I thiught that when you take your insulin before eating that it would drop your bg in preparation for the food you are about to consume. I took the NovoRapid at 7:47 and the bg reading was 4.4/79.2 then my mother phoned with an "urgent" message (you try interrupting your mother!) and at 8:13 -on a whim just to see what happens, it was 5.0/90. So what happened? Anybody have any ideas? Thanks