So, no one thinks Ill Girl is about to die? Anything shy of that is okay, given that's in the realm of possibility here.
It's possible but, maybe mostly optimism on my end, the overall mood of the manga has been uplifting and relatively light. The heaviest drama we've encountered were Mashiro finding out how they first met, the Love polygon with UdaXRiriXPresXNurse, and that (adorable) "fight" about the beach. Nagisa seems to know the severity of Mashiro's condition/her actual diagnosis since her hesitation about the beach was about not knowing what to do for Mashiro, not because she believes Mashiro's illness is fatal and could worsen due to the trip.
Besides, Mashiro's fear is losing Nagisa so it seems more like she's anxious about their future in a way that doesn't imply her death. Mashiro's aware that her condition makes her heavily codependent, needy, and a little selfish. They could momentarily part so Mashiro can seek treatment to better her condition so she can gain independence. There's also Nagisa's tendency to become hyper-focused on her goal (which might be becoming a doctor so she can help Mashiro) that can cause some friction between them. Hopefully the foreshadowing is just misunderstandings or even arguing because Nagisa's too busy with cram school.
But again, it could just be me wishing the ominous cloud doesn't necessarily mean instant death flags.