I'M SO WORRIED!!

I am currently 8 weeks pregnant and have just received word from my booking in blood tests that I am rubella non immune.i don't understand this as I was immune in my first pregnancy a year ago.i am so worried about this as I cannot get a vaccine untill after baby is born and I am petrified of contracting the virus and harming my baby.how is it even possible to know if someone has the virus? can someone shed some light on my situation or give me reassurance that it's not as serious as I am thinking?