I'm actually the same way. There have been a lot of deaths in my life, a few in my family, but I've not mourned over one since I was 5.

Still, I understand being upset over the death of your mom. I hope you can find some brightside to it. Or at least appreciate her as much as you can while she's with you. It will dull the pain.
Be glad that you know. It may seem like torture to know, but it's much worst to be completely blind sided.