Folliculitis or Infected Sebaceous Cyst: Given the description of a nickel-size pimple with a pinpoint opening at the center, which appeared and possibly "popped" a week ago, folliculitis or an infected sebaceous cyst is a common and plausible explanation. The presence of mild itch and the absence of significant pain or drainage support this diagnosis.

Acne Vulgaris: Although less common in the area under the breast, acne can occur anywhere on the body where there are sebaceous glands. The description could fit a large pimple or acne lesion, especially if it has come to a head and then "popped."
Eczema (Dermatitis): Eczema can cause red, itchy patches, and if it becomes infected, could resemble a pimple or have a similar appearance to the described lesion.
Keratosis Pilaris: While typically presenting as smaller, rough patches on the skin, keratosis pilaris could potentially cause a larger, inflamed lesion if irritated or infected.

Breast Cancer: Although extremely unlikely given the acute onset and description, any new breast lesion warrants consideration of breast cancer, especially if there's a family history or other risk factors. The absence of pain and the presence of a pinpoint opening make this less likely, but it cannot be entirely ruled out without further evaluation.
Infectious Abscess: An abscess could present similarly, especially if it has drained or is about to drain, indicated by the pinpoint opening. While the patient denies drainage, an abscess would require prompt medical attention to prevent spread of infection.
Necrotizing Fasciitis or Other Severe Soft Tissue Infections: These are rare but life-threatening conditions that could initially present with skin lesions. The lack of severe pain, fever, or other systemic symptoms makes this less likely, but vigilance is always necessary.

Pilar Cyst: These are benign cysts that originate from the hair follicles and can become inflamed or infected, presenting similarly to the described lesion.
Epidermoid Cyst: Similar to sebaceous cysts but lined with epidermal cells, these can become infected and present as a red, inflamed nodule.
Hidradenitis Suppurativa: A chronic skin condition that can cause painful, inflamed lesions in areas of the body with apocrine glands, such as the underarms or under the breasts. The lack of significant pain and the acute onset make this less likely.