Can she be pregnant?

My girlfriend and I had with a condom but the condom slipped off inside her. She had her period the next day and also started birth control pill. She also took a pregnancy test the day after the first day of her period and the results were negative. She is still paranoid that she might be pregnant because she said her period was shorter. I think her period was shorter cause of the birth control pills. Should she take a nother test or is she safe since she is now on birth control pills and she had her period?