I saw this comment prior to my reply above.
I am sorry. Let me add a comment. I know that, normally, syphilis lesion is painless, but I have read some articles saying that it is painful sometimes in anus (and difficult to differentiate from hemorrhoid). I am very worried.
Welcome to the forum. Thanks for your question.
Syphilis is not a likely cause of your anal symptoms, for several reasons. First, it is not likely to be transmitted by rimming. While that probably could occur in theory, most likely it is very rare. Second, condoms are highly effective in preventing penile-to-anal transmission. Third is the nature of your symptoms -- not only the pain, but the absence of an obvious open sore; and syphilis is not likely to cause anything like "swollen veins". Fourth, the symptoms started somewhat early: although 2 weeks is possible, 3-4 weeks is more typical for onset of a syphilitic chancre.
Gonorrhea also is not known to be transmitted by rimming and of course condoms are highly effective in preventing it.
Further, as you apparently know, your description of "swollen veins" with pain is highly suggestive of hemorrhoids -- which is a common problem in all people and is especially frequent in those who have receptive anal sex. And I disagree that it is difficult to distinguish syphilis (or gonorrhea) from hemorrhoids. The symptom could be similar, but not the appearance of the anus.
It would be a very bad idea for you to take antibiotics without professional evaluation. Don't do it. While it may be "difficult" for you to seek medical care, that is exactly what you should do.
The bottom line (no pun intended!) is that this is much more likely to be hemorrhoids, perhaps triggered by somewhat traumatic anal sex, than it is syphilis, gonorrhea, or any other STD.
I'll be happy to comment further if you will return to let me know the outcome after you have been professionally evaluated. In the meantime, I hope this information has been helpful.
Regards-- HHH, MD