Watching someone you love suffer is the hardest thing in the world, and I completely understand why your mum is putting on a brave front, doesn't matter that your an adult, you're still her child and she wants to protect you. My parents were the same when my mum was ill, they made light of it, which I suppose was the best thing for a while because for quite some time I thought my mum would be ok, eventually found out the truth and the worry and fear was just so overwhelming, so I understand why they kept it from me. Your mum likely thinks she is helping by acting like things aren't so bad, she's doing it for you.
If you need to find out exactly what is going on then talk to her doctors and/or nurses, they will be able to tell you more.
I know its very hard, but you will get through it I think in these kinds of situations we find an inner strength we didn't think we had, we carry on because well we have to. You dont have to be strong all the time though, and no one can really cope in these kind of situations, we can get through it, but it does hurt, it hurts so so much.
Losing my mum hurt alot, and still does, but I'm getting through it.
I hope talking to your mums doctors/nurses helps. Sorry you are going through this.