It sounds like you probably have a condition that is known as "dermatographism." Essentially what this is a predisposition of the skin to develop inflamed red marks when it is stimulated by scratching. What happens is that scratching stimulates certain inflammatory cells in the skin, call mast cells, to release chemicals that cause local redness and inflammation. Generally speaking, this condition is very benign and, in fact, it is extremely common. It does not usually require any treatment, other than trying as best you can to avoid scratching the skin and causing the red welts to appear.
Sometimes, however, dermatographism can be associated with more serious allergic problems. For example, if you notice that you are routinely breaking out in red welts all over your body, even without scratching, then this is something that will need to be investigated. Also, if you have any other allergy symptoms, such as swelling of the face or wheezing, then this is more serious.
What I would recommend is that you set up a visit with your primary care doctor
. They can perform a physical examination and make a diagnosis of dermatographism. They can also make sure there is nothing else going on that would require more extensive evaluation or treatment.