Potty issues

Question
I have a year and a half old black lab who I have had for about a month now. I adopted her through a private party and she's been great except for one thing: she poops in the house when I leave.
She is potty trained and does not urinate in the house ever...she goes through the night and a few hours throughout the day without having any issues at all. Even if she has done her business in the morning, when I come back 2 hours later from class, she has gone again in the house. It seems like a separation issue perhaps, but if that's true, how do you solve that?

Thanks!

Answer
I'm a big advocate of using a dog crate.  I tell people it's like using a playpen for a toddler.  It keeps them safe and keeps them from causing any destruction.

If you are not familiar with how to use a crate, I am including information here:

http://www.ddfl.org/behavior/crate-train.pdf
http://www.hsmo.org/m_obedience/behavior/dogcrate.html

Problems of separation anxiety usually manifest more in the form of barking, destruction of the home, self-mutilation (licking themselves raw), etc.

http://www.ddfl.org/behavior/sep-anxiety.pdf
http://www.marinhumanesociety.org/Behavior/Handouts/SeparationAnxietyShortRx.pdf
http://www.marinhumanesociety.org/Behavior/Handouts/Anxiety.pdf