You don't subconsciously "know" anything. My mom has been health anxious for her entire life. She's 71. I'm 31, been at this since early childhood, still alive. It's anxiety and obsession, not any kind of true instinct.
I'm sorry about your friend. It's always hard when someone we love dies, and harder still when they're young and we're unprepared. But it has nothing to do with your own health problems, perceived or real.
I'm sorry you have to deal with all the things left unsaid, but she probably knew how you felt. My friends don't always say things I know they think or mean - when my friend was diagnosed with cancer I took the time to email a bunch of people I'm closest to, thank them for being in my life, etc, because I'd been given that wake-up call. And I got really nice responses in return. But while the things people said were nice to hear, none of us were saying things the other didn't really know. If you're good friends with someone, especially if you've known them for a while, you just know things like that.
It might make you feel a little better to get in touch with the other important people in your life and tell them the things you'd wish you'd told them if, god forbid, something happened to them or you. I know it helped me a lot.
I would also, again, recommend a grief counselor or therapist. This is a hard thing to go through, HA or no.