Sudden Itch and Rash?

I went on a long hike last weekend. Towards the end of the hike and insane itch developed on my back around my upper waist area, under my bra line but not towards the middle of the back.
A relative rash was noticed, nothing severe.  
Once I got back to the car I used ice from the cooler on the area and it went away, itch and rash.

Now I would like to know what happened, I'd hate for it to happen again.

I was sweaty of course, though it was not a new deodorant actually one I've used for years, nor a new detergent. It was a relatively new bra that had been washed but not all cotton.

Thank you for your input, this really put a damper on an otherwise great hike!

[BhumikaMD]

It could be eczema, sweat dermatitis, allergic reactions or eczema.

You need to wash the areas several times with fresh water. Do not use any cosmetic products at the sites.

Apply calamine lotion at the site of the lesions and see if it helps. You could take some oral antihistamine medications like cetrizine or loratadine. You need to maintain a good personal hygiene .

Anti-itch drugs, often antihistamine, may reduce the itch during a flare up of eczema, and the reduced scratching in turn reduces damage and irritation to the skin.

For mild-moderate eczema a weak steroid may be used (e.g. hydrocortisone or desonide), whilst more severe cases require a higher-potency steroid (e.g. clobetasol propionate, fluocinonide).

Eczema can be exacerbated by dryness of the skin. Moisturizing is one of the most important self-care treatments for sufferers of eczema. Keeping the affected area moistened can promote skin healing and relief of symptoms.

http://en.wikipedia.org/wiki/Eczema

Let us know if you need any further information.

It would be advisable to consult a skin specialist for your symptoms and a proper clinical examination.

Let us know if you need any other information and post us on how you are doing.

Regards.