Two lovers meet in the forest, away from everyone, and the speaker slips in a detail that changes the mood: the man he lies with is someone's husband, or the woman is someone's wife. The wording is ambiguous. "His wife / a pretty woman" is a strange, almost offhand remark, as if the speaker is half admiring and half guilty, noticing the person who is being betrayed.
The meeting is quick and hidden, and the poem moves quickly too. Then morning arrives with "blood / and fleas," which strips away any romance. The forest isn't a pleasant bed, and the body pays for the night with bites and scratches.
The last line, "me / and not me," is the most revealing. The speaker seems split, unable to recognize himself or herself after what happened. It could be shame, or it could be a sense of having briefly been someone else. The mood is uneasy and bare: desire, secrecy, and a faint self-estrangement, all told in very few words.