Call the doctor and describe the situation - is she holding her breath or can she not breath - very different things... but...
My daughter would hold her breath if she hurt herself badly and got really scared. She first did it around 12 months when she was learning to walk and bashed her head on osmething. It kept up until she was 2-2 1/2. Twice, she held her breath until she passed out.
The doctor assured me that as soon as she passes out, she will start breathing again. I just needed to stay close and comfort her and catch her if she fell, so she wouldn't get hurt. It is VERY scary, but the doctor was right. And I sort of learned to predic if she would do it, and I would grab her, shout her name, and pat her back firmly which usually distracted her enough to catch her breath.
Just remember to never shake her, and inform her caregivers - one of the day care teacher broke down in tears after my daughter passed out in front of her.
Enjoy her - my baby is now 5 - I can't believe how fast it goes!!