Many times the general std screenings they do are not comprehensive. They only screen for a few out of the hundreds of std's that are out there. Honestly, it doesn't sound like you have the classic symptoms of a bladder infection, but that's something that you should definatley get checked out by a doctor.
A bladder infection can take a couple of days to show itself, and it most certainly does not occur immediately after sex (i should know, i'm one of those lucky girls who is prone to uti's after sex).
Anyways, please go see your gyno and ask to have a more comprehensive std test. Given your symptoms, your doctor can do a variety of tests for different std's that could be the culprit.
It could be an uti, but given the symptoms and time of onset, I would say probably not. But then again i'm not a medical doctor :)