A bit OT but I had something strange happen with my Rubella immunity and this has reminded me to ask my Dr about it tomorrow!

I fell pregnant with my DS in March 09 and had my immunity checked around April. I had borderline immunity. I was last vaccinated against Rubella when I was 12 (so 16 years beforehand).
Anywho, I didn't do anything about my borderline status as I'd thought I'd wait until I was no longer immune to worry about it.
I'm now pregnant again. Had my immunity checked again in November '10 and I'm suddenly immune... What the!? I definately haven't been vaccinated since and I definately haven't contracted Rubella in the meantime. I am really, REALLY interested to know why I suddenly have immunity. Seems very, very odd to me.