My mom says my newborn has a cold?

My mom is helping me take care of my twin girls who will be 4 weeks tomorrow. She was fussy earlier and when she sneezed she let out a giant booger (gross, sorry!) She doesn't sound congested to me or anything, and I told her I don't think she could have caught a cold this early on, since supposedly she should have antibodies from birth. Should we be worried just because she had that booger or should I just wait until she exhibits more signs of a cold? What should I be looking out for to determine that she does have a cold?