The events, from Jennifer's perspective at any rate, are rather clearer in the film's official novelisation.
Due to the change in the timeline (caused by Marty not having an accident involving the Rolls Royce) Jennifer only now half-remembers seeing the fax come through. When she finds the paper in her pocket, the message on it disappears entirely just at the same time as her memories of the previous night, which now no longer happened, start to fade away. This explains her confusion.
This was more than just a dream. There was a piece of paper in her
pocket, a paper she remembered.
She pulled out the crumpled page and looked at the huge letters at the
There! It wasn’t a dream after all! It was—
The words disappeared from the page.
‘It erased!’ she exclaimed.
‘What?’ Marty asked, looking at the page over her shoulder. ‘What
Jennifer frowned. Why was she staring at a blank piece of paper? And
why had that paper been in her pocket?
‘I don’t know,’ she admitted. What importance could a blank piece of
paper have, anyway? She was with Marty, now, and they were going to
have a wonderful time together. That was what was really important,
When she encounters the Doc, with a head full of questions, a memory made out of swiss-cheese and absolutely no answers, she shows him the disappearing message note hoping that he'll explain to her why it's so important. His answer is not especially forthcoming.
‘Then it wasn’t a dream,’ Jennifer muttered at his side. He looked
over to see her staring intently at a crumpled piece of paper that
didn’t seem to have anything on it. She looked up at the inventor.
‘Doctor Brown, I just want to know one thing What happens to Marty and
me in the future?’
Doc considered her question for a second before answering. ‘In the
future? That’s up to you. Your future hasn’t been written - no
one's has. For better or worse, your future is what you make it.’ He
winked at both Marty and Jennifer ‘So make it a good one. Both of