Itchy rash on back and chest --what is it \ why am I getting it?

Many possible causes. Best to get evaluated to get accurate diagnosis but possibilities include folliculitis, grover's disease (wax/wanes often worse in hot weather), drug reaction.
Many possibilities. There are many types and causes of skin rashes. Several types of inflammatory conditions of the skin, allergic reactions, and sometimes bacterial, viral or fungal infections can cause rashes on the skin. Insect bites can also be the cause of a rash on the skin. Have it evaluated by a professional and get the appropriate treatment.