Red blotchy warm skin on chest?

Many possibilities: There's a long differential here, from seborrheic dermatitis to allergic or irritant dermatitis to a yeast or fungus infection - hives - something called erythema multiforme - the list goes on and on. A lot depends on how long it's been present, whether it itches, its precise appearance, etc. You really need to have a dermatologist look at it in person.

VisualDX: Hi, it’s hard to say without a picture but there is a great app that may be able to help you get a diagnosis without being seen in person by a dermatologist. The app is called VisualDX and is one of the best, if not the best out there. Many clinicians, including myself, use it. Just snap a photo and it gives you a list of possible diagnoses. Good luck!