After getting a glimpse of the future at the end of This Is Us season three, one of the biggest mysteries this season has been figuring out why everyone is going to see Rebecca. Though the flash-forward never outright reveals what is wrong with her, it's assumed she's suffering from some sort of memory loss since she appears to not know that Randall is her son.
However, we've been getting more and more clues about Rebecca's condition this season. After dealing with memory loss, Rebecca decides to go see a doctor. Following a series of tests, we eventually learn she has a "mild cognitive impairment." Though the diagnosis doesn't exactly reveal what's wrong with her in the flash-forward, an MRI test later confirms she is showing early symptoms of Alzheimer's.
Though creator Dan Fogelman and actress Mandy Moore initially debunked the theory that Rebecca has Alzheimer's disease, we're almost certain now that they were just trying to throw us off. While Rebecca has a hard time retaining new memories, she's also remembering events from the distant past (like her marriage to Jack) in vivid detail, which is a sign of the disease.
If this is the case, the flash-forward is going to be that much more heartbreaking. Not only could Rebecca forget that Randall is her son, but she might forget that Miguel is her husband. Perhaps that's the reason Nicky is at her bedside in the flash-forward? She might actually think he's Jack. We are so not ready for that.