<human>: EDIT: im 22M, woops!!

My girlfriend and I have been together for 6 months (a year in total, 4 months casual dating, 6 months dating). I recently found out that while we were casually dating she had been seeing and having sex with other people. We had been seeing each other for four months and she had met up with this person (the most recent i know about) a month before we were "official". She then continued to meet up with him 
right up until the day we were "official". This included having sex with me and him on the same day and texting me as she would leave his house in the morning after staying the night for sex. This also included getting angry at me for not wanting to come round in the afternoon  of that day.

I'm at a loss as i feel like i was deceived and had no idea this was happening. When i had asked her if she was seeing other people at the time she had said no. And i wouldn't have started going out with her if i knew what she was doing.

   

What should i do?

Give me the gist of the text in a nutshell
<bot>: Current girlfriend seeing other men during 4 month courtship until we were official. Included having sex with me and other men on the same day a week before we were "official". I had no idea and am at a loss.