Should I be concerned?

my wife is out at least 5-6 evenings per week including friday and saturday sun and her explanation is she needs her space. she is sometimes gone for more than 4 hours returning at 11-12 and on several occions 2-4 am. she seldom answers her phone and when asked why we are not able to reach her she says she did not hear it ring or she did not get the calls. her phone is always lock and when at home she seldom move arround without it. She say she is - sitting in her car relaxing or having a cup of coffee or took a friend and her husband to the specialist. Befor her mid life crisis she did not stay out late only if we go tigether, now she seldom does any thing in the evening with me or the kid, right now she is out since 1 pm may return at 11 pm or so. morning text messages are explained out as the phone company. What is really happening here- any ideas-