The most common cause of genital sores is herpes. However, you don't provide enough information for me to really say whether it's likely in you. When you get the sore, how does it start? How long does it last? How often does it happen? That is, what does "it comes and goes" mean?
Recurrent genital herpes usually starts with a red bump that over the next 2-5 days develops into a blister-like or pimple-like lesion, then opens up, then gets a scab, then heals, total duration 10-14 days. This almost never happens more than once a month, and between outbreaks everything is entirely normal.
But all I can do is speculate. You need to see a health care provider at a time when the sore is "active", ideally when it's just a day or two old. Although some urologists are up to speed on herpes and other STDs, many (most?) are not. A family medicine doc or dermatologist would be the best bet. Or visit your local health department STD clinic for the best chance at expert care that will be highly confidential.
In response to your last question, people with genital herpes are at higher risk of catching HIV if exposed. But still, with common sense and safe sex, avoiding HIV is easy even for people with herpes.
Good luck-- HHH, MD
Hi,
It is BigLoom. I just wanted to add some additional text.
I have been tested for STDs which may cause joint related problems and all the tests came back negative (I had joint pain and doctors ran bunch of tests to check for possible causes - do not remember what STDs they checked for - eventually I was found to have arthritis and this was determined to be the cause of the joint pain).
With regard to my comment that my biggest concern is that I have a STD which puts me at greater risk of getting other STDs, what about when a girl gives me fellatio? Suppose the sore is present or not present when a girl gives me fellatio - does this put me at a greater risk of getting a STD from that girl? This is a huge concern of mine.
Thank you
Receiving a blowjob is incredibly low risk, sore or no; around 1 in 20,000. No worries about that.
But it is a remarkably bad idea to receive oral when this sore is present--not because of the risk to you, but because of the potential risk to your partner. Like if that sore really is herpes, you could give your partner oral herpes. Go show your doctor. I swear, it's nothing s/he hasn't seen before.