Pls i've had a reddish, swollen sore on my labia. why and what should i do ?

May be Herpes: You have not mentioned if this sore goes away and comes back or is it for how long, neither you have mentioned if it is painful,l or painless. Therefore due to lack of all tat information I will guess that you may have Herpes Progenitalis or syphlis or some other form of STD ( sexually transmitted disease) . Please a doctor as soon as you can