Hi,
You might be experiencing chaffing, and/or razor burn (that is, if you shave or wax) along your bikini line. Chaffing can be caused by sweat or moisture that doesn't dry quickly enough, and constant friction. Try wearing cotton underwear when possible, and use an anti-chaffing powder or gel (sold at most stores in the medicine aisles). Sometimes sporting goods stores carry anti-chaffing products also. If you don't see any improvement soon, have it checked out by your doctor.
Good Luck :)