Is this really cheating on my girlfriend/should I tell her/she flashed me on webcam?

I'm currently in a long term relationship with the girl that I love. Being a little desperate the other night, I was on chatroulette and a girl came on. Not even thinking, I asked her to flash me, and she did. As soon as she did, I quit the conversation not believing what I had just done. I feel guilty and not sure if I should tell my girlfriend. I know this may be "cheating" but I know its not nearly as bad as like for example having sex with another girl, or actually meeting a girl. This girl that flashed me was totally random, and I have no clue who she is or anything nor do I ever plan on doing this again or ever meeting someone else outside our relationship. What should I do?