If you have been tested as you said and they ruled out infection, perhaps you aren't lubricated enough during sex. If not that, then if you have unprotected sex, his sperm could also be an irritant to you.
It sounds like you have a UTI (urinary tract infection). If not treated, it could become much worse. You need antibiotics. You can also stop at the drug-store/chemist and ask for some 'urinary alkalyniser' / in Oz it's there's a good one called "Ural". That will help the burning to go away, drink plenty of water. And definately go and get some antibiotic therapy. This is an acute infection and needs to be treated soon. You will be feeling like new within a couple of days.