Erythema Toxicum Neonatorum (ETN) or Acne-like Eruption: Although the child is 7 months old, the description of erythematous macules/plaques that appear and disappear, especially in the context of nursing, could suggest a benign condition related to feeding or environmental factors. However, ETN typically presents in the first few days of life, making this less likely but still a consideration for a similar-appearing rash in an older infant.

Contact Dermatitis: This could be due to an allergic reaction or irritant from something the child comes into contact with during nursing, such as detergent residue on clothing or a new soap.
Eczema (Atopic Dermatitis): Although eczema typically presents with more chronic symptoms, an acute flare could appear as erythematous macules/plaques, especially if triggered by an allergen or irritant during feeding.
Seborrheic Dermatitis: This condition can cause a rash on the face and trunk and might be exacerbated by feeding if the infant's position or the mother's clothing causes friction or irritation.

Child Abuse: Although unlikely given the description, any unexplained rash, especially one that appears and disappears, should prompt consideration of non-accidental injury, particularly if the pattern of the rash suggests a specific object or method of infliction.
Infectious Etiologies (e.g., Scarlet Fever, Kawasaki Disease): These conditions can present with rashes and are critical to identify due to their potential for serious complications if not treated promptly.

Allergic Contact Dermatitis to a Component of Breast Milk: Extremely rare, but could theoretically occur if the mother consumes something to which the infant is allergic, causing a reaction during nursing.
Neonatal Lupus: A rare condition caused by the transplacental passage of autoantibodies from mother to fetus, which can present with a rash among other symptoms. However, it typically presents in the first few months of life and is associated with other systemic findings.