Frodo's Dream

[Faenaduial]

Here is the quote I think you were referring to:

He lay down again and passed into an uneasy dream, in which he walked on the grass in his garden in the Shire, but it seemed faint and dim, less clear than the tall black shadows that stood looking over the hedge.

I'm not sure the bad dream was caused by the wound; remember, he has been chased through the wild by the Black Riders and that should be enough to give anyone bad dreams. But I do think it very telling that he feels the Shire is growing dim and the black shadows are clearer. I always thought that was caused by the wound. Remember Gandalf told Frodo when he was recovering in the House of Elrond that he was beginning to fade.