A number of conditions can cause large red blotches on the skin. According to MedlinePlus, you should have any new or unusual skin changes checked out by your doctor or a dermatologist to make sure you don't have something that could lead to serious complications. While many skin discolorations disappear in time, even with no treatment, call a doctor if the spot starts bleeding on the skin or if the discoloration does not go away within a few weeks.
When blood vessels underneath the surface of the skin break, they can form large red and purple bruises. Purpura occurs when the blood vessels leak and cause a large red blotchy patch that can be seen just under the top layer of skin. Tiny spots of purpura are called petechiae and large blotches are called ecchymoses. Certain drugs, fragile blood vessels or congenital disorders can cause the blood leaks.
Many different kinds of rashes can cause large red blotches on the skin. Contact dermatitis occurs when your skin comes in contact with a substance to which you are allergic or that has certain chemical properties that react negatively to skin, such as dyes, soaps, latex or poison ivy. Seborrheic dermatitis usually develops as a large red rash around the mouth, nose, trunk and eyelids or behind the ears. Stress, age, extreme weather conditions and oily skin can cause the red splotches.
Illnesses such as lupus, juvenile arthritis and Kawasaki disease often present with large red skin rashes. Other types of skin disorders that may appear in the form of large red blotches on the skin include eczema, shingles, impetigo and psoriasis. Insect bites can spread and leave a large red discoloration. Childhood illnesses such as chicken pox, scarlet fever, measles and roseola often result in red splotches.
While exposure to the ultraviolet rays of the sun can cause melanoma and other dangerous skin cancers, other symptoms can occur from sun exposure that lead to large red blotches on the skin. According to the American Academy of Dermatology, blisters and large red blotches occur if you are allergic to the rays. You may break out in large red splotches if certain medications you're taking interact with the ultraviolet rays. Common medications that often cause adverse skin reactions include antibiotics, birth control pills and medicines that treat depression, high blood pressure or arthritis.
When sweat gland ducts become blocked and perspiration becomes trapped underneath the skin, large red splotches can begin to form. According to MayoClinic.com, heat rash can affect anyone, not just babies, especially during hot humid seasons. In addition to discoloring the skin, heat rash, also called prickly heat, can be itchy. You can get heat rash from intense exercise, wearing heavy lotions or creams that block the sweat ducts, or anytime you become overheated. Certain medications also can cause heat rash.