What are the chances my girlfriend is pregnant?

Patient: Hello,I am concerned my girlfriend is pregnant. Let me give you a little background. My girlfriend has always had very irregular periods (28+ days in the cycle). She has also been on the pill for 2 1/2 years.We had her last period June 1st and hasnt bleed since. We last had sex 4 weeks ago. She has told me that she has not been feeling any symptoms of pregnancy whatsoever and is telling me not to worry. She also took a pregnancy test 3 weeks ago and it came out negative. She has been taking her pill almost flawlessly for the last 5 weeks and has only missed one. I am very concerned. What is going on?Since last time we had sex was 4 weeks ago and she has not had any symptoms and she has been very good about taking her pill, I think she may not be, but the fact that she hasnt bled in forever is concerning to me.