I had unprotected sex with a girl 3 weeks ago and 2 weeks ago my arm pitts started to hurt. They aren't swollen but they hurt. I tested negative for every std, but I don't knowe what's wrong with me

Lymph nodes are located in many places in your body, one under each armpit. They are the body's warning system of a low grade infection in the body, or any kind of infection for that matter. So there is something going on with you.

Did the doctor doing the test have any idea what they thought ?
Did the doctor treat you with a general antibiotic for infection ?

If the unprotected sex was only 3 weeks ago, then there really hasn't been enough time for any STD to incubate... your tests would come out negative. I think you should be tested again and also talk to a doctor about the pain.

If the unprotected sex was only 3 weeks ago, then there really hasn't been enough time for any STD to incubate...your tests would come out negative. I think you should be tested again and also talk to a doctor about the pain.

I like the get tested again. But three weeks is more than enough time for a STI to show. Whether is dependent on the health and magnitude of infection.

We are shooting in the dark if you don't say WHEN you were tested and what ELSE you were tested for, and what your HISTORY is (presumably that wasn't the first sex of your life). To connect swollen lymph nodes to just 1 of any number of causes without a full exam is ludicrous.