'Self Sex' and Fantasizing About Another Woman - Cheating?

Hi All,

I have recently been having a great deal of anxiety about something and I wanted to get other guys opinions.

I am currently in a relationship with my girlfriend who I love, and I would never in a million years cheat on her.

Recently while masturbating, I actually fantasized about another woman while watching porn. This is somebody who I had past sexual experiences with, but do not (and won't) come in contact with in my current relationship. Hypothetically, if any situation presented itself with this woman, I would never act on it, or even consider it, and I have no feelings towards this woman. It was strictly a fantasy which I used during masturbation.

I have been experiencing anxiety about this lately, somehow convincing myself that I "cheated". Obviously, if my girlfriend knew this was my fantasy, she would not approve. One part of me says that it's cheating because of the hypothetical lack of approval, and the other side of me says that it was strictly a fantasy, and it was not real. It was simply in my mind.

I wanted to get other guys thoughts on this, including your experience. Thanks.