Had my menses on2/05/14 and then had unprotected sex on 19/05/14, is there a chance that i may be pregnant??

Maybe: If you usually have a 31 day cycle, then you probably ovulated on day 17 (May 19). That would make your risk of conception about 25%. However, each cycle can be a little different. So there's a chance you ovulated earlier or later this cycle, and were fertile on a different day. I would take a pregnancy test to be sure. They are accurate about 14 days after conception.