This is actually meant to confirm the doubts that Lewis Bodine had about her actually being Rose Dewitt Bukater. Lewis and Brock were arguing before Rose and her granddaughter arrived about the authenticity of Rose's story and who she really was.
LEWIS: She's a goddamned liar--Some nut case seeking money or
publicity. God only knows why, like that Russian babe, Anesthesia.
Rose Dewitt Bukater died on the Titanic when she was 17, right?
BROCK: That's right- If she had lived, she'd be over 100.
LEWIS: Okay, so she's a very old goddamned liar. Look, I've already
done the background on this woman all the way back to the '20s when
she was working as an actress. An actress! There's your first clue,
Sherlock. Her name was Rose Dawson back then. Then she marries this
guy named Calvert. They move to Cedar Rapids and she punches out a
couple of kids. Now, Calvert's dead, and from what I hear, Cedar
Rapids is dead.
BROCK: And everybody who knows about the diamond is supposed to be
dead or on this boat, but she knows.
Yes...Rose was over 100 years old and more than likely suffered from some sort of short term memory issues. However, this particular scene was more of a light comic relief confirming the suspicions of Lewis.
After Rose arrives and is unpacking, Lewis and Brock go to check on her. This is when Rose has her short term memory lapse. Immediately following her shrugging off her comment, we see Lewis roll his eyes to Brock in somewhat of an "I told you so" manner.