I see what you're saying, Pom Pom, and I agree. The first impression we had when Cheryl told Brendan that Nana had phoned her was that she really was in her final days, but the reality once we saw her was somewhat different. She was working, despite having to use oxygen, and she obviously knew her days were numbered but I think the urgency for her was that there were things she needed to say and do, and she knew she had to do them while she was still capable.
When she had to go to hospital it was for a chest infection, obviously connected to the cancer because her resistance was weakened, but not the cancer itself. I didn't find it all that hard to believe.