Having with an ex after you've had with your partner during the get to know you stage? Cheating?

I love my girlfriend. She sees me as her world and everything in it she would give to me. I met her two weeks before she moved away to college for a semester. We got to know eachother at a rapid pace and made a very memorable impression upon one another. The first weekend of her moving away I drive to visit her where that weekend we also shared our first ual experience. She had come out of a year and a half relationship before finding out her boyfriend was gay and had very strong and conclusive reasons to believe that. It had been 3 or 4 months since they parted and was not ready to move on but took a strong liking to me. We're now approaching our 5th month together and i've learned plenty from her and her from I. Yesterday she called me crying on the phone saying she needed to speak to me. We sat in her car and confessed she slept with her ex to fill voids of him being gay days after i went up the first week and we had for the first time. Is that cheating? We were not official.