ok. The first thing that comes to mind, in the presence of itchy spots on the genital area is scabies. However scabies is contracted from another person and usually spreads around. Though the lack of lesions elsewhere or anyone else in the family is unusual in scabies, I would still suggest treating for it, given your sexual encounter a month ago. Pubic lice are another possibility, but treatment is the same as scabies.
The best way to treat scabies is with permethrin cream. You can get permethrin 5% lotion over the counter. However everyone in the house needs to apply the cream head down to the toes, twice, a week apart, as per instructions in this website below.
In addition, to help with the itch, I would recommend the following:
1. Calamine lotion applied 3-4 times a day
2. Cortisone cream applied twice a day
3. Benadryl tablets 6hrly as required, to stop the itch.
This is nothing serious and should clear up with treatment.
Hope this helps. Do keep me posted & get back if you have any further questions or feel that I have missed out anything. I will be here to provide ongoing advice as necessary. Please do not hesitate to ask. Best wishes.