Hello,
You have not mentioned the description of the rash but since the
rash is spreading and from the area described, it can be due to
contact dermatitis, allergies, heat rash, sweat dermatitis, fungal infection or pityriasis rosea.
Pityriasis rosea is a common human skin disease which presents as numerous patches of pink or red oval rash. The rash may be accompanied by low-grade headache, fever, nausea and fatigue and itching. Topical and oral steroids are needed.