By "bikini line," are you talking about the top line where a bikini would rest, i.e., the whole "waistband" of a bikini bottom? Are you talking about the top of the pubic hair, where some women shave so it won't show when they wear a bikini that is low cut?
If you're talking about only where the top of the pubic hair is, have you been shaving? Could you have an allergic reaction or a reaction to dye in soap or whatever you use when you're shaving?
If you're talking about the whole waistband, same question about allergic reactions, but I'd ask if you have recently gotten new underpants that might have some kind of dye or elastic to which you are sensitive?