Thanks for your query,based on the facts that you have posted it appears that you have red pimples at the tip of your penis .
These pimples could be due to inflammation of terminal part of urethra
and external urinaru meatus secondary to acidic vaginal secretions of your female partner who is likely to have vaginal infection
Please get her examined by a qualified Gynaecologist .
Since you do not have STD proved on screening test you need not worry at all.
You need to take broad spectrum antibiotic like Cefexine along with anti inflammatory drug like Diclofenac
twice daily.along with topical antibiotic
ointment like Neosporin
Ensure to wash your genitals with warm water twice daily
Ensure to avoid sexual encounters till it heals up completely.