What strikes me from your post is the fact that your Mother had cancer for just over a decade before you were informed, and apparently other than that basic knowledge, you are not permitted access into the more intimate details that we sometimes need to help us better-understand what our loved ones we are going thru. I gravitated to that point (not in judgement, but from commonality), because as a caregiver myself, I've entered into a world in which that theme is more prevalent than I'd have imagined. I'm lucky enough to have a loved one who so far has kept me completely informed, yet I also attend caregiver groups in which the opposite scenario is discussed by members who are in a similar situation as yourself.
As mentioned by /u/Rampaging_Elk, It's your Mother's situation to manage and the info she shares, she does so at her own personal level of obligation. That is her privilege. But it should also be acknowledged that while it's the individuals right, that doesn't exclude you or any other person from feeling pain and/or sadness at not being allowed in, and you should feel ok to feel that way. Both parties rights and feelings can be true, even if they conflict to some extent.
I don't say the following to judge any party one way or the other, but I thought it might be comforting to know that many others go thru what you are going thru; the frustrating sadness/anger/confusion/compassion/hopelessness that often comes from the not-knowing and/or possessing of, the needed criteria to help alleviate some of the pain being left out of the loop can cause as you consider the person suffering.
Whatever the future holds for you, I again point out to agree with the sage words from 'Rampaging_Elk'
That being said, something sounds off. I'd press her a little to make sure she's alright. Give her a chance to open up, and show that you want to help if you can, that she doesn't have to just be tough for you. Don't approach it like a scared kid, not wanting to lose his/her mom, but approach it with her feelings in mind. Try to get closer to her.
Couldn't agree more. Good luck to you.....