it could well be hormonal, either that or that area may just get a little more natural oils produced and clogging in that area. do you find that you sweat more in this area?
its just natures way really, try exfoliating the area once a week (that facial wash with little grains in it) it scrubs out the dead skin cells and any dirt in the area, when its all refreshed dab on some aloe vera to soften it and to rebalance the moisture.
you can pick up a simple scrub/exfoliater and aloe vera anywhere, very good prices!