it's probably due to excessive friction. you're either most likely allergic to spermacide is you use it...or latex on the condom...which i hope you use b/c it's well worth it. if you don't use either, then you could just be drying out and that's why the vag is all itchy and irritated. get some lube!
If you get it after you have sex, it can't be herpes. What it is ranges from irritation to maybe an allergic reaction to something you're using.