Why does my 25-day pregnant dog have mucus coming from her vagina?

My boston terrier is approx 25 days into her first pregnancy. She has started showing normal signs of pregnancy (breasts enlarging, weight gain, lethargy) but today I noticed a small amount of yellowish mucus coming from her vagina. Is this normal? I know dogs lose their mucus plug before going into labor, but its not close to time for her have the puppies.