well.. if your a female, it could be a UTI, if your a male and had sex with a female that had a UTI at the time, then you could have a really bad infection, or if you are writing this after having sex, then your an idiot, it will burn when you piss right after sex because the mix of urine and sperm create something that burns your insides bad, but not a big deal to be honest, just a burning sensation, now if you have not had sex in a while then i would say go to the clinic and get a checkup done on "mini-me" because you may have an infection
EDIT: again with the sex thing, you could possibly have some sort of Sexually transmitted disease, such ashttp://www.google.com/aclk?sa=L&ai=CnnBx5CgFTNm2JYevnwfMjpCWAbSs3bkBlPDy3xP02tG5QwgAEAEgtlRQmNKp1P3_____AWDJtqyI9KPAEMgBAakCB7_Cfh5OuD6qBBZP0P6fy4iOaplS3yDFsuQPDfL7h0cGgAWQTg&sig=AGiWqtwuGd3szrT7prm2fCN5rYTpBizDPw&adurl=http://www.thelocalchemist.com/index.php%3Fdispatch%3Dcategories.view%26category_id%3D427 or Gonorrhea