Hello,
The bumps appeared the day after you had intercourse and hence the possibility of irritated hair follicles and folliculitis is more rather than any STD. Apply warm compresses, Neosporin ointment and calamine lotion and avoid messing with it. You can take anti-inflammatory pain killers like ibuprofen or acetaminophen. If the lumps persist then it will be best to get it evaluated from a dermatologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Well , it is a possibility but unlikely. You may want to get checked out just to be sure. Oh by the way, to test for certain STD's in male patients, they have to stick a swab up your penis. Just thought you should know. I doubt you have a STD as the vast majority of STD's do not show up immediately or in the few days after sex.