Thanks for your query.
As your Dermatologist
told this does not seem to be herpes
or genital warts
and as your tests for Syphilis
were negative you just forget about these diseases.
It could most probably due to yeast infection or may be due to some irritants like condoms ( latex or due to lubricants), soaps or other topical applications.
If it is not adequately treated an yeast infection, it may recur. Then your sexual partner's are not treated for the same then also you may likely to have the recurrences.
So take your sex partner also to your physician and treat her, then you will be alright totally.
Get well soon.