I really don't know how to give you help but I think I can offer a little bit of insight.
Sometimes with PTSD, absolutely ANY emotional event- positive or negative- can trigger bad feelings.
For example, I am an athlete & last year I made it to our National competition, which I had worked my butt off for. But when I got to Nationals I couldn't stop crying & I felt horribly depressed. I got angry with myself for not being able to enjoy anything, which turned to me being angry at other people.
The way I reason it in my head is that trauma hits an emotional soft spot that doesn't get used very often. When poignant things (in your case, your child's birth- congratulations by the way ;)) happen, they can come close to the epicenter of the trauma related feelings & they can get directed at an inappropriate event.
Sorry if this is hard to understand!