If you have sex with other men frequently, you should test at least once per year. There are men that do it every three months. That depends on yourself. But, whether you choose one option or the other, make sure your sexual encounters are always protected. Ask you casual partners if the use condom in their intercourses, and avoid to have sex with those ones that don't respond or say that they don't use it. At the same time, change the condom as many times as possible, to reduce the risk of getting it broken and use as much lube as possible.
Another interesting thing you should do is getting the vaccine against Hepatitis and HPV.
All the best.
As skin in that area is really sensitive, it could be just because of the physical action of penetrating. It could also be an allergic reaction to latex or lube.
But oral sex exposes you to syphilis, gonorrhea and chlamydia. Although those spots are not typical symptoms from any of those STDs, I would strongly recommend you to test. I will let you know how to do it:
-3 weeks after the encounter, test for gonorrhea.
-6 weeks after the encounter, for syphilis, and in case the result is negative, repeat it 3 months after the encounter, as there are people that take longer to show a positive result.
-3 months after the encounter, test for herpes.
All the best.