Yes I started to feel a lot like you and found myself getting very down about it. Then when DS was 6 months old he became very poorly and we were almost certain he would die. Luckily he pulled through but I was still plagued with thoughts of "what if he had of died?" And "what if he gets run over by a bus/suffocates under his covers at night/any other accidental death scenario". It got to the point where I would sit and plan his funeral in my head and I'd be in floods of tears just thinking about it.
I really had to take a step back and put my rational head on. I've had a lot of CBT counselling in the past so I drew on what I had learnt from that and applied it to myself. Now if the intrusive thoughts pop back in to my head I'm quick to squash them and think of something else. No good comes from imagining the worst.
That's my experience anyway. Have you spoken to your DH about the way you're feeling? It's not good to keep thoughts like that locked up as your mind can blow them all out of proportion. Death happens to everyone, it's a fact of life. Just enjoy your baby while he's still a baby, those days soon fly past!
Since I gave birth to my DS I've become increasingly concerned about my own mortality and that of my DH, my DS and other family members. The thought of death is making me utterly furious, particularly the thought that someone's existence could be ended just by accident. I feel suddenly like I'm on a clock and that I've just been wasting valuable time for years. Plus I am a little preoccupied with pain (possibly due to a crappy birth experience). Has anyone else's view of death/mortality changed since the birth of a child? And how has it changed you? I'm interested.