She has started working lots of overtime
A classic sign of infidelity is when your girlfriend starts working more--a lot more.
Dr. Sommer notes that this suspicious change in work schedule can manifest itself in different ways: leaving the house earlier to go to work; coming home later; or claiming to have many business dinners.
But take note: is there a pattern to these extra working hours? Every Tuesday and Thursday, for example? If so, that may be a sign that something is wrong.
Keep in mind that if the job is new or she has recently received a promotion, there is no need to immediately suspect that she is cheating.