This is a major Red Flag. Honor the threat. Read the standard of evidence post Standard Evidence Post
Stay calm, do what you can to appear trusting and strive to be a little more cheerful and helpful at all times. Few serial cheaters get caught by a rookie mistake like this one. But boy do the first time WS learn quick. So keep him off guard. This is a genetic list of red flags, and to be honest learns more towards WW then WH but see if any applies
Sex life dropped off noticeably
Passwords on phone and computer
Much more time on line...fakebook.
More GNOs...staying out later.
Less eye contact and holding hands...much less physical contact.
Staying up late on computer.
Dressing more provocative
3 hour groceries shopping trips
Gasoline use and mileage went up
Generally disconnected from family.
New friends that I wasn't introduced to
Cell/text usage went up...way up.
New hair style and attention to makeup
Started exercising more.
Secretive about whereabouts during contact
She would become annoyed easily with me.
Household responsibilities dropped way off.
ecame more forgetful in general
A noticable distancing from her family.
Much more waxing...trimming...shaving....not for me.
Hat tip @chap