That just happens sometimes. I tend to get a bit daft if I go below 5, but shortly after I discovered Candy Crush (evil time-vampire...) I was playing it on the bus, and felt nothing til I got off the bus and felt a bit shaky. Tested and my BG was 1.9. I had no idea my meter could measure that low! Of course once I knew I was hypo I started to feel it in a big way, I'm sure it has psychosomatic elements.
But try not to worry too much, hypo symptoms change all the time and sometimes you'll miss one - that's why we have BG meters