My rubella igg came out positive and im 19weeks pregnant. is this mean that i had measles in my first 3weeks of pregnancy? my igm is non reactive.

Unlikely: The IgM will eventually drop off a month or two after primary infection so absence at 19 weeks doesn't tell you much about early pregnancy. Unless you were ill with rubella during that period I would not expect it.This pair of tests are often done in pregnancy and your results is what they expect from someone who received the vaccine as a kid or had the disease before pregnancy.