I don't really have much to add to what others have said, except to emphasize that you may need to go over even if she's "not feeling up to it" if she's withdrawing. It could be depression, or grief, or physical illness that are causing her to become withdrawn, but no matter what it is, this is when she needs you most.

I had a really rough time with depression once and really withdrew from my friends. I have a special place in my heart for the friend who just kept showing up. I felt incapable of connecting with other people, making the effort to be social, etc. and at the same time I felt very abandoned by my friends. One of my friends just wouldn't let me go - she would come to my place to show me something she was knitting (we've always been knitting pals), or to "visit a friend" who lived in my house, or just because she hadn't seen me in a while. She didn't abandon me. She didn't have any expectations of me - we could knit together, or talk, or just sit, or whatever - and I always knew I had a friend who cared. I really can't overstate how much her steadfast friendship helped me.