Could my girlfriend be pregnant

[Pleasehelpme6991]

My girlfriend is on birth control and she had been on birth control for 2 months when we last had sex. We didn't use a condom so she took plan b along with birth control. She was supposed to have her period two weeks later but didn't. We thought it was because of the hormonos. So we waited for the next month and she continued to take her birth control. She took 3 pregnancy test in the morning during that month and they all came out to be negative. She didn't have her period. So we waited another month and she continued with her birth control. She took two more pregnancy test and till said negative. She is supposed to start yesterday  according to her birth control but has not yet. She has not been feeling nauseous or being peeing more than the usual. She does not feel any differences in her body. Just stress