My ds used to do this all the time. It was like he was upset and took that big breath in to scream out but he just couldnt exhale properly. The first time he did it DH wasnt home and i freaked out big time so i know how you feel. What always worked for us was either a loud hard clap of the hands right in his face - it kind of startled him into taking a breath or i would splash water onto his face. The water was the last resort but worked a treat. Once he didnt breath for so long he turned completely blue and went limp in my arms and i started mouth to mouth on him. First breath and he came good but dear me that was the worst ever, i still feel paniced thinking about it. We have had him fully checked out and they couldnt find anything wrong. Apparently my dh used to do the same when he was a baby. Ds hasnt done it since about 15 months old. Big squishies for your scary moment