I woke up today and found small brown spots in a cluster, about 3 inches big, on my forearm near my wrist. this literally happened overnight.diagnosis?

Impossible to tell: For sure without actual examination. But if is sudden onset and localized to a 3 inch area, it is most likely something that happened to your skin locally. For example, contact dermatitis, insect bite, local irritant. If it spreads to other areas of the body, you must look to systemic disorders. Go to dermatologist if it persists.