My 2 and half year old was waking up a little in the early morning and then he got really stiff…

My 2 and half year old was waking up a little in the early morning and then he got really stiff seemed like he wasn't breathing I'm not sure if he was or not it only lasted a few seconds, I already called his pediatrician but they haven't called back yet. I am wondering if anyone has seen or heard of this before? it's very scary