Thank you for the picture.
This does not look like an STD. Even though you have not had sex in 5 months, it is possible for certain STDs to appear months after an encounter, but this does not look like any STD.
This looks most like an area of irritation and inflammation that most likely has occurred because of friction. This can happen from sexual activity or masturbation, but only when there has been a recent episode. If there has not been any recent episode, then it may have occurred because of friction against clothes or bed linens.
It is also possible that it could be an early yeast infection, as a yeast infection can occur without sexual activity, but a yeast infection would usually have some itching.
At this point, the usual approach would be regular cleaning twice daily to promote healing, and it would usually heal over several days to a week or so.
If I can provide any additional information, please let me know.