I just started getting a rash under my breast, it also has s funky smell to it. What can I buy or use to get it to go away?

Yeast. This is probably a yeast infection, called candidiasis. Keep the area clean and dry, and apply an anti-yeast cream (several are available over the counter). If that doesn't work, it's either resistant to the otc-strength creams, or is something less common. Either way, you'll need to see a dermatologist. But try the anti-yeast cream first.