Mine was really low (ie zero) during my pg with Charlie and my OB told me to get the vaccination done after I had had Charlie, but I never got round to it, and then fell pg again, so totally freaked. Basically if your immunity is low, the risk is that you may contract German Measles, which can in turn have an impact on an unborn baby, especially in the early stages of a pregnancy, from memory, things like miscarriage, still birth, brain damage, deafness, vision problems, or heart problems. This is called Congenital Rubella Syndrome. The risks of any of these things are highter in the first tri, & reduce in the 2nd tri, and are minimal in the 3rd tri.

I am not sure though, whether you can get the imunisation if you are BF.......if I were you, I'd get an appt with your GP to ask their advice.