About a week ago I had unprotected sex with a guy who came inside me. Since yesterday, my skin has been itchy, my joints have been sore in some places, my muscles have ached a bit, and my stomach has been upset (I don't feel like eating, but I do eat...). Earlier today, I had diarrhea for the first time. Obviously I need to go get tested, but if I go now for the p2 antigen exam, will it be accurate? Can these symptoms happen only after a week or 10 days??

A week to 10 days is fast but possible. You should get tested now-- If you are HIV antibody (ELISA) negative, a p24 antigen test can be done. People who are diagnosed and treated during acute infection appear to have the best chance of maintaining a normal immune system according to some experts. And you should have follow-up antibody tests at the right intervals. Hopefully, your symptoms will all be due to anxiety, but don't take any chances again if you are really HIV- this time around. It isn't worth it.