What Causes A Missed Period With Negative Pregnancy Test?

Hi, I am 21 years old (male) I am asking this question on behalf of my girlfriend 22 years old f(emale). she is aproximetly 10 days late for her period, this has happened before but not for this long! last time she was about 3-4 days late and then she eventualy came on. she has been on the pill since she was about 15-16, we usualy use protection i.e. condom but 2 weeks ago we had intercourse without using one, she took a pregnancy test yesterday which clearly stated negative, she has been feeling tired and bloated since she has missed her period, one thing i can say is that she has been stressing about money alot recently and also she has a personal trainer that she sees once a week so i'm not sure if it's because she's been on the on the pill for too long? stress? excersise? or maybe something to do with weight? thankyou for your help in advance