Cracked or dry skin could mean just that, or it could mean a fungal infection, ezcema, dermatitis, or a host of other things. It doesn't sound like an STD.
However, since you've had unprotected sex, you should be screened for STDs (most of the time, people don't get symptoms from STDs and don't know they have them), and let your doctor see this spot. If it's fungal or ezcema, or something else, they can treat that, too. Don't try to self-treat. If you're wrong, it could make whatever it is worse.