I had protected sex 3 days ago. Right after sex, I had a rugburn on my labia minora. The rugburn has now turned into an open sore. Is it an std?

See your doctor. These are symptoms that require a thorough evaluation by your doctor. Only after obtaining this informstion, can he/she adequately diagnose and treat you.
Could be. You need to see a doctor to have an exam. What you describe as a "rugburn" could just be irritation that now has infection, but it could also be a sexually transmitted disease such as herpes. Either way, this needs to be looked at so it can be treated!