Period and Negative Pregnancy Test Is Pregnancy Still Possible?

My girlfriend has had back pain for over 2 months now. She said she has herniated discs but of course I had to start worrying about if it was pregnancy related. She has had 3 periods since one time where the condom slid half off. She is on birth control too. I told her that I was afraid she was pregnant and she thought I was crazy. She got freaked out though and when her period was hours later than usual she took a pregnancy test yesterday. It was negative and her period came an hour later. I for some reason am still worried about some crazy even in which she would only have back pain as a symptom and have periods and a negative test. Is this any possibility or should I stop worrying?