Is she pregnant?

had protected with female friend on 19 Nov - almost a month ago. i did not c*m and the condom did not break either. well she thought her period was supposed to start at the end of the month or first days of this month but never had it. so since the beginning of the month she has taken three pregnancy tests (her most recent being tonite) and they all come back "not pregnant"! is it safe to ume with a higher degree of certainty that she is not pregnant? i know she is having financial problems and I wonder if that stress is leading to a delay in her period. please let me know your thoughts....FYI, please spare me the "moral" and "ethics" comments. Just asking for your health opinions and not your political leanings. Thanks in advance and good night :)