Don't panic.
Something similar happened to me many years ago, and it turned out I had a cold (and didn't even know it).
Apparently my body had produced so many white blood cells to fight it off, it skewed the results of a blood test. Also a nurse took my blood pressure and thought I was about to have a heart attack!
I went back 2 weeks later to get re-tested, and everything came out fine.
So, in all likelyhood, it's probably just that that something weird had affected you that day, like a cold, or something you ate or drank etc. Or maybe they screwed up testing the blood sample.
In any case, get another test done, and see what that says.
It will probably come out fine, but if it doesn't, then you need to talk to your GP about what could be causing the low reading, and what can be done to resolve it.