If this appeared after applying some lotion, then it can be an allergic reaction
.Other common cause o fsuch rash is fungal infection
, but in that case generally there is itching.
You can try ---
Nonprescription (over-the-counter) remedies include
• anti-itch creams containing camphor, menthol, pramoxine (Itch-X, Sarna Sensitive), or diphenhydramine
• antihistamines like diphenhydramine (Benadryl), chlorpheniramine
(Chlor-Trimeton), or loratadine (Claritin, Claritin RediTabs, Alavert); and cetirizine (Zyrtec).