<human>: For the sake of getting too specific, as this is a throw away, I recently discovered my girlfriend lied about who her most recent ex was.  
When we first got together she said that she left her ex roughly January 2015 and that was the last time she had sex. We got together in November 2015. However, I was being nosey on her facebook account and noticed she has a life event of her being with someone else in May 2015 (This is hidden from public view and only she can see it). Honestly I wouldn't care as this is such a short time from this person to me but she has mentioned him before as they were friends for awhile prior to her breaking up with the January 2015 ex.  
They still communicate somewhat via text, FB, etc etc, but not constant communication. I'm sure it was a quick fling that seemed like a good idea but ended up not being so. However, it's the lie that has me bothered. When there is a story that involves him, she refers to him as her friend. 
Am I over-reacting? Should I confront her? (being civil and not aggressive of course)

Summarize the given text in a few sentences.
<bot>: Girlfriend claims her most recent ex is from over a year ago but I know she dated someone between then and me.