The speaker describes feeling like a tower built from borrowed material, something tall and imposing that doesn't really belong to itself. The "borrowed matter" suggests a body and a self assembled from things that came from elsewhere: other people, other days, the past. Then comes the sickness. The speaker is "ill with all yesterday," as if the entire past had been swallowed and refused to settle.
The last lines bring the idea down to earth with a small, almost wry comparison. The speaker's head aches from the wine drunk today, the plain, familiar misery of overindulgence. By setting a big feeling beside a petty one, the poem makes the weight of the past feel both enormous and oddly ordinary.
The mood is queasy, heavy, and tired, but touched with dry humor. The title, "Malaise," fits: it names a vague unwellness that is part physical, part spiritual. The poem doesn't explain what is wrong. It simply lets the reader feel that dull, sour fog of carrying too much of what has already happened.