<human>: I will go into more detail when I wake up and am in a better "place" so to speak.  Long story short, my girlfriend that I have dated for 3.5 years and I broke up.  I ended it with her (however it was mutual) after consistent lying about lots of little things.  The final instance was when she went to a bar on Wednesday night and came back at 12:30 and just said that she had been studying and her phone wasnt working.  Stuff like that.  We are in college together, so it's not like its a live-in thing, however, we obviously spend the night together alot.  

Two weeks after the break-up, she is already with another boy, and they are sleeping together.  Whether or not they have sex is something I am not sure about just to be clear.  This girl is not one who was ever "easy" or anything like that.  She is very conservative with her morals which I always respected her for.  Seeing her do this has really upset me just because it is not something that I expected her to be doing at all.  I think that is why I am having alot of trouble in the first place is because it is just so unlike the girl I thought I knew.  I just have no idea how to even begin to explain how I feel; besides the usual stomach consistently in a knot.  I just really need help understanding what to think and what to do in the situation.  I have no idea why she is doing this.  

Females, what are y'alls guesses? Males, what would your reaction be?  Someone earlier told me "there are other fish in the sea" and I responded with "Yeah but she is my nemo." All Im saying is I am not going to go hook up with other girls to try and get back at her or anything.  I just really need some help understanding all of this.  I feel so lost.

Can you provide a brief rundown of the text in within 3 sentences?
<bot>: Girlfriend of 3.5 years is sleeping with a boy already after we just broke up.  It is tearing me up on the inside.  What do I do?