Hello,
It can be due to contact dermatitis, allergies, fungal infection, heat rash or sweat dermatitis. Avoid using any lotions and creams for the meantime.
Use a mild cleanser with moisturizing properties like Cetaphil. Also use cotton clothes and change the clothes twice if there is any increased sweating. Try keeping the skin free of sweat as far as possible. If the symptoms still persist then you can apply mild steroid like dermacort. For the itching antihistaminics like Benadryl or Claritin can be taken.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Sorry forgot to add basic personal info and where it started and spread. I am a 28 y/o female in Michigan. History of allergies to everything (cats, dogs, dust, mold, grass, trees, and several medications. I have hypothyroidism, migraine headaches, muscle spasms in back, neck, shoulder and hip from car accident., obesity, acid reflux, and high blood pressure (with migraines). It started on my low back, neck, upper arms, and upper pubic hair. It is still at those locations and has spread to the rest of my back, shoulders, chest, under breasts, belly button, buttocks, entire abdomen, under arms, sides, thighs, crease of inner thighs, and back of the knee. HELP!!!!!!!