Anywhere on the body where there is bloodflow can get a hickey, including your penis and testicles.
As CurfewX says, I think it’s possible to have a hickey anywhere on the body. However, normally you’d be aware of very strong ‘sucking’ which creates a vacuum & therefore the hickey, but you didn’t mention if there was anything unusual during the oral sex. Was there some very strong sucking during this?I think the thing to watch for is whether this goes away within a few hrs or persists. It’s been many yrs since I had one (!) but it seems to me the hickey usually turns purple after a day or 2 & then gradually clears up. If this spot doesn’t diminish significantly within a few days, see your Dr...
You wouldn't see symptoms of any STD this quickly. The absolute minimum for seeing symptoms of an STD is 2 days.
Receiving oral sex does put you at risk for getting gonorrhea, syphilis, NGU and genital hsv1.
You can test for gonorrhea and NGU at 5 days after exposure. (NGU means non-gonococcal urethritis, or any infection in the urethra that isn't gonorrhea.)
You can test for syphilis at 6 weeks, though syphilis isn't that common.
If you don't already have hsv1 orally (think cold sores, fever blisters), then you can get it genitally if the person giving you oral sex has it orally. You can test now with a type specific IgG blood test to see if you already have it, and again at 4 months to see if you got it from this encounter. If the person giving you oral sex didn't have any visible sores on their lips (herpes is on the mouth, not in it), chances of you getting it are lower.
Its not something you need to freak about, or run out and test. If you are sexually active, you should test regularly, and can test for these the next time you test. Oral sex is lower risk than genital or anal sex.