Ah ok, I thought there was a chance you might be low in that vitamin, as some people don't supplement with it (which is a bad idea as lack of B12 can cause nerve damage amoungst other problems).
Did you manage at all to increase your weight? I remember when I was young I only had my period when my weight was at least of a BMI of 20.3. People's build (I mean bone structure) and fitness level can mean their minimum weight needs to be a bit higher than someone else's to get a regular period.
Anyway I hope you get it sorted out soon