Unless these were enormous flowers, I suspect you were relatively close to them, and therefore nowhere near the edge of the image circle.
Ultrawide lenses just aren't the best choice for macro work, where DOF is short anyway. A better choice would have been a macro lens or an enlarging lens (unless you have much more bellows than I would have guessed from your initial choice of a 75mm lens). It can even be a lens that doesn't cover the format at infinity, like a macro lens for 35mm or medium format or an 80mm enlarging lens. At macro distances, lenses for smaller formats will often cover 4x5".