I guess the safest thing to do is take a few sitzbaths in something good for you like a vinegar solution or boric-acid solution (keep them mild -- mostly water) and give it a week or two and see if everything heals. In the meantime, keep your fingers off, since the hands can transmit bacteria. Then later when you get a chance, get tested for STDs, since it sounds like the sex was unprotected. I don't think you would get a rash from an STD only 3-4 days after the sex (though I'm not sure about some conditions, like herpes) but it really sounds like too much friction and not enough lubrication.
How are you feeling now?
Since you had symptoms right after sex, it doesn't sound like an STD. Herpes symptoms don't appear for at least 2 days, usually 4 days, but can take much longer.
If sex isn't comfortable, it's really important to speak up. "Bearable" is not the goal - pleasure is the goal. You may need extra lube, which is perfectly normal. It doesn't mean you aren't aroused, as loads of things can affected how much lubrication your body produces - nerves, hormonal birth control, where you are in your cycle, condoms, alcohol, etc.
Have you and your boyfriend talked about STDs? If you aren't his first partner - including oral sex - has he been tested?
All of this means having some possibly uncomfortable conversations, but the better the communication, the better the sex. :)