It’s definitely the parasympathetic kicking in, but whether that response is a ventral or dorsal thing is hard to say without being there. If it feels like suddenly an emergency brake slams on, then it’s probably the dorsal shutdown response. Another possibility is that your system is more used to being activated than it is to being calm. When we have been through trauma we can actually get addicted to that high stimulated state, and it is calming down and NOT feeling our heart pound that freaks us out!