That was the only thing that confused me the first time I saw it, but the second time it kind of made more sense. It was more than 100 days since they saw each other, and people have started relationships and been married in shorter time periods than that. I guess that kind of supports what she said- she wasn't sure enough to marry Tom.
I saw it again a few weeks ago, and it was a a bit sadder for some reason. I think knowing what happens to the characters puts it into a completely different perspective.