Sorry to hear that your second child has had a bumpy start to life. If you had been patients of a critical care team in a hospital, contact them about how you are feeling now, and check out if they have any after-care support for mums like you, or can - at the very least - signpost you to other support services, in real time, or online social platforms, so you don't feel so isolated.
At the time, all the focus was on your little one and ptsd, in very simplistic terms, is when you have received a jolly big fright and your body is in total 'safe' auto-pilot mode, in that you get up, you go to sleep, you might eat, you follow instructions and you hope like hell that the Worst does not happen, while fearing that it might. The experience you have gone through is being on the edge of a cliff 24/7 with your body turbo-pumping adrenalin to get you through it.
Having endured all that, when the immediate danger is over, and your whole being feels that it is safe to come out of the shell (the storm has passed), that's when the mind starts to register the enormity of what you've been through, the ways in which it's impacted you (trauma) and with two little kiddies relying on you, there is more pressure to not drop your bundle and keep going. You start to run on fumes and that can become a vicious circle : the more you force yourself, the more drained and burned out you feel, until you are chasing your tail.
Give the hospital a tingle and take it from there. Hope you find some lovely new mum friends who 'get' where you are at in the here and now.