Hello and thanks for posting,

Rashes are not common signs of breast cancer, and would usually affect one breast. Also, breast cancer is rare in someone of your age, so try to not worry about this. You can get misleading information from google, as it does not replace a consultation with a pharmacist or a doctor. 

You very likely have a common skin rash, probably a sweat rash. Do ask your local pharmacist for a steroid/antifungal cream to apply. They can advise you further. If it doesn't clear up, then consult your GP, as sometimes prescription creams are needed.

Hope this helps,

Vanda