Sudden Interest in Work

• If your girlfriend previously disliked her job and now seems excited to go to work each day, it could be a sign that she's particularly interested in whom she will see there. You might also notice that she's begun making a stronger effort to dress nicely, perhaps even buying new work clothes, as well as making sure that her hair and makeup are done perfectly each morning.
  
  

"Working Late"

• Someone who is cheating with a coworker may begin spending extra time at work, or using a heavy workload as an excuse to leave earlier than usual in the morning or return late at night. She may begin claiming to have to work on weekends in addition to keeping erratic hours during the work week and hiding her pay stubs from you to cover up the fact that she's not working all the long hours that she's claiming.
  
  

Lack of Invitations

• Accompanying your partner to work-related events is a normal part of any serious relationship. If your girlfriend used to invite you to go with her to holiday parties, company picnics or other work functions but those invitations have since stopped, it may be a sign that she would rather appear single and spend quality time with a certain coworker at these events.
  
  

Mentioning Someone Frequently

• While normal workplace friendships are no cause for alarm, if you notice that she is mentioning a certain person's name more frequently than anyone else's, this combined with one or more other issues may be a warning sign of infidelity. If a woman is romantically interested in someone, she is apt to talk about him more than usual without even realizing it.
  
  

Secretive Behavior

• One of the most common signs of any type of affair is sudden secretive behavior. A cheating girlfriend may begin leaving the room and speaking in a low voice when talking on the phone, deleting emails and text messages, and quickly closing out of windows on her computer when you enter the room. However, it is important to remember that even if your girlfriend is displaying some of these signs, it does not necessarily mean that she is cheating. Be careful and consider the consequences before making any accusations.