A red rash suddenly appeared in my chest the size of a tea-cup. please advice?

Many things --> Rash: Thank you for your question. An eruption on the skin could be caused by a number of things. There are different types of inflammatory conditions of the skin that could cause a rash as well as responses to external sources such as sunlight or other ultraviolet exposure. Having an examination performed by a qualified medical practitioner would be helpful in diagnosis and treatment.

Bite: Without seeing it a sudden large rash could be a variety of things, insect bite, allergic reaction, infection to name a few. Please post a picture or come see your doctor as soon as possible so we can see what the rash looks like.