Postpartum Depression

In May of 2014 I lost my son to PROM and a Prolapsed Cord. He was due in October.

I've now for a few months been babysitting a little girl that could've easily been the same age because we need the money (I got diagnosed with POTS shortly after and had to give up my job). She's adorable, and I have a lot of experience with babies. I just love her. Sometimes though, she's completely inconsolable. I've noticed lately that after at least 20 minutes of that behavior, I end up either in tears or just angry. I'd NEVER do anything to hurt a child, but I get so scared. Is it possible that I'm having PD symptoms with someone else's baby?