Mommy Guilt

I'm going to the movies with friends after work today. The last movie I saw in the theatre was WALL*E with my 4yo. Today we are seeing The Secret Life of Bees. Though I do leave the house for 9 hours a day to go to work, other than a quick stop at the fabric store or a run to Target on the drive home, for the most part I am in the company of my son. Granted, a portion of that time consists of me behind the sewing machine and him playing in the same room because, well I need to pay the bills! Weekends are commited to him and his brother (my stepson who stays with us on Sat/Sun). Evenings out with friends are few and far between, maybe once every few months. My parents take care of my son while I'm working and when I do make "grown-up plans", they watch him then too. I have no other options for childcare. My husband & I have been separated for over 3 years and he lives out of state, so having my son stay with him is not in the picture. And since I live with my parents, hiring a babysitter would be silly and awkward. My parents have never said anything about my going out. I give them advance notice, call when I'm out, make sure things are taken care of, etc. So why do I feel so guilty going out once in a while? I don't go on dates, I go to work and I go home and I take my son with me most everywhere else. Is this just a single mom thing or do happily married moms who leave the kids home with their husbands get this guilty feeling too?