As you are guessing, itching in the pubic area could be a sign of sexually transmitted diseases (herpes, syphilis, gonorrhea, chlamydia), but if you are not sure that you are at low risk for these conditions (keep in mind many of them can lay dormant for a long, long time, only to develop symptoms later), I would recommend making an appointment with your primary care physician
for an exam and sexually transmitted diseases screen.
Sexually transmitted diseases aside, itchiness in the groin area can be a symptom of tinea cruris (also known as "jock itch"). Tinea cruris is a fungal infection of the skin of the groin. It is caused by spread from other areas (tinea pedis, or "athlete's foot") by applying the same towel to both areas. It is also worsened by excessive moisture: failure to dry off completely immediately after sweating or showering can cause the fungi to grow as they thrive in damp, dark, warm places.
For now you should make sure to dry off as well as possible before putting on your underwear. You should be seen by your primary care physician or dermatologist
to make sure there is nothing more serious going on. If it is, in fact, tinea cruris, anti-fungal creams or sprays can be prescribed.