Hi GHT,
I think you're doing the right thing, witholding the information about your health, knowing that if you did share your news with your Mum there would be even more stress to deal with. This is stress you just dont need right now (or ever), you need to concentrate on yourself and getting better, not having to deal with someone elses crap! Its a wise decision, but also a hard one too, but call it self preservation!
Perhaps your good friend is totally shocked by your diagnosis and not sure how to be. If she's never had an experience with cancer maybe she's just lost for words and feeling a bit awkward, so instead she says nothing...but nothing hurts, hurting both of you in the end and your long friendship. Go for a coffee and have a real open and honest chat, explain to her how your feeling, she'll probably be so relieved to have had you open up to her, that she'll too explain why she's been a bit distant lately.
My Dad has pancreatic pancer which is terminal, and while we've known about it for over a month now, I havent told many people about the sad news. I think I want to deal with it myself first, so then I can face all those concerned faces out there, that right now I can't face.