My Doctor Said That I Had A Heart Murmur. Since

My doctor said that I had a heart murmur. Since I got pregnant that's the time that I easily got tired especially when I'm walking and short of breath... But when I got rest back to normal for what I feel.
And I'm worried about it. Tuesday I'm going to meet the technician or the cardiology.
My question ma'am / sir what is the possible cause of getting tired I'm 18 weeks pregnant now and I'm 41 years old.
Thanks