Yes, your googling has already told you the possibilities - irritation, friction, maybe it's a fungal infection, etc.
If you used a condom for oral sex, it's not an STD. No STD gives you a red spot that lasts for months, in any case. Even if the oral sex was rough, the redness from it wouldn't have lasted this long.
Are there any bumps or lumps? Does it feel different to the touch than your other skin? Has the redness gotten bigger?
It could be the way it rubs against whatever underwear you wear, so maybe try and adjust yourself, and see if that helps. If it doesn't, getting a doctor to take a look would be a good idea.
If it's itchy, painful, the redness grows, or it changes in any way, for sure get a doctor to check it out.
how much dose it tuck in I use baby oil sometimes it helps alot