What are the chances of my girlfriend being pregnant?

Last May about the 17th my girlfriend got her period. We've had sex on a few occasions and now she hasn't had her period since. Her periods have always been some what irregular but not to this extent. So it's been 37 days since her last period. When she didn't get her period we got a pregnancy test on the 20th of June. It tested negative. What are the chances it was wrong and that she is pregnant. Our current plans are to get another test, but are we just getting paranoid? Is it normal for a girl to not her period for this long?

TL;DR: My girlfriend had her period May 17 and hasn't had it since. We took a pregnancy test around June 20th and it tested negative. What are the chances this is wrong and she is pregnant?

Edit: Yes condoms were involved 100% of the time.