I have a reddish pimple-like sore on my labia majora. I was wondering what it is?

Likely a cyst . It is possible that you have a small sebaceous cyst. These usually are asymptomatic unless they become infected or become really large. Also bartholin's cyst can occur near the entrance of the vagina. Cysts usually occur from blocked ducts causing secretions to build up. Best thing to do is to see your doctor if it does not improve or resolve in 2 weeks.