sounds like maybe she did lose her mom. I lost mine 5-30-10 and am having a terrible time. Wish I had a good friend like you for support and help. Your friend is very lucky.

just let her know that youre there for her. maybe you can pop by for a small visit. you dont have to talk a lot, but just having someone there is nice. (sometimes she will want to be on her own, and thats ok too) let her know youre there for her even though she doesnt reply to your calls or txt. she will come around eventually, and then she will need you more than ever. grieving is a very difficult time. not only for the person herself, but also for the people around her. cos its hard knowing what to do or say, and like you say, you dont want her to think you dont care. or that youre being annoying. i dont know but my advice to you is to let her know youre there. send her txts even when she doesnt reply.
when my bf died, i didnt feel like talking to anyone either. i also replied to peoples txts with just one word. i didnt care about how it made them feel, cos i felt so bad myself. looking back, i really appreciated the ones who didnt "give up" on me. the ones who kept txting me even tho i rarely replied. the ones who let me know they were thinking about me, even tho i didnt care about anything but feeling completely broken.
you sound like youre a really good friend, and im sure you will know what to do and when to do it.

good luck x