Honestly, I think he was just a bit rough with you. You probably weren't incredibly wet or maybe he went a little too hard with his fingers. The clitoris is extremely sensitive, and if you touch it directly without enough foreplay it can be downright painful. Same with stroking it directly up and down, that is pretty much always painful, turned on or not, for most women. Try asking him to start out lighter and go in circles or side to side with his tongue/fingers. And I find lube helps a lot. I used to have the same problem with my boyfriend, he is a sweetheart and always wants to make me happy but sometimes he can just get too into it. I have slowly gotten more comfortable with telling him what can be painful, I felt bad about it at first because I knew he was just trying to please me but a swollen clitoris can be EXCRUCIATING and annoying and can last for a few days depending. Just don't do anything with your boyfriend (down there) until the pain goes away, then try it a little gentler. cheers ;)
Your boyfriend doesn't have to have an outbreak or symptoms in order for him to pass on anything to you, so it can be possible that you have an STD. Has your boyfriend had sex before? And has he been tested for STDs? It might be absolutely nothing, and could very well be your pants that caused the sore, but I would talk to your boyfriend about what he's done before sexually. It's not something you want to risk, if he might have something that you can get. good luck.