(((((((Amy)))))))
Your mom sounds so much like my dad. The silent treatment is such a demoralizing thing to have to deal with from a parent, and like you, I always felt I had to be the one to make amends and make things better, long after I'd forgotten the mortal sin I'd committed to create the mess in the first place.
Instead of making amends, maybe you can just say to her something like, "Mom, I'm sorry you're pissed off, but I don't feel I've done anything to deserve this attitude." Try saying something that will acknowledge her mood but keeps you from taking responsibility for it. I don't know if it will make things better or worse, but it has you really upset and you might feel better by trying to get things out in the open. Maybe if you have your say, you'll feel better about it, regardless of her reaction.
I completely understand how you feel. When I stopped taking it so personally, the silent treatment used to really **** me off. Oh wait a sec, I never stopped taking it personally...
Sheesh, you'd think they'd grow out of it!
Hugs,
JG