I've been NC for a month and I feel like I cheated?
if you feel "guilty" for sleeping with someone when you're no longer with the person you perceive to have "cheated" on, it's because you are still emotionally attached to your ex.
consciously, your understanding will tell you the relationship is over and you are nc. go deeper. your emotional mind is still connected to your ex, even if you're physically no longer together.
guilt is an emotion. one that is there to inform you that you may have done something wrong. you didn't. which means, a part of you still "feels" as though you are in a relationship with your ex. you're not. this points to your attachment towards her, emotionally.
move towards detachment. this state of mind will allow you to release the attachment you've created and prevent the feeling of guilt to continue.