Girl Issue

So I guess I'm making a notorious TL girl problem thread...

I've got a girlfriend of 2+ years who i go to university with and it's been mostly pretty good. One thing about her - she extremely friendly. Borderline flirtatious with just about any male she meets, even if she doesn't do it consciously.

(semi-relevant paragraph)
A couple of months ago, she started talking again with her last boyfriend who during the course of our relationship, she had never spoken to. They talked on the phone, but mostly just texted - texted like 2 kids in junior high just starting puberty. I told her it bothered me, and she said she wouldn't text him as much. We're on winter break now, so I've only seen her once over the past couple of weeks. When I did, it seemed like she had stopped texting him. I was suspicious though and checked her facebook PM inbox because she has her login saved on my laptop. She was now just facebook messaging him instead.

Today she told me she had been at a party last night and said she fell asleep as someone was talking to her. Her story was kinda oddly specific and strange, so I looked on her facebook inbox and saw that she had made up the story to cover for hooking up with a guy at the party. I would guess that they just made out or something, most likely not sex. This is just some random guy, that she is semi-friends with.

This isn't the first time either. She admitted making out with a random guy last year as well.

I don't know if she'll tell me without me confronting her. I won't be able to just wait around and see if she does, I wouldn't be able to pretend to be nonchalant...

Should i just end it? Give her an ultimatum?

update:
Well i confronted her. First i asked her if the story she had first told me was true, (about her falling asleep for 15 minutes) and she just acted confused and didn't really say anything. Then i eventually told her i had seen the FB messages and that she couldn't be telling the truth. Still, she vaguely denied anything happening that she hadn't told me about.

She "explained" that the guy was confronting her about his own girl problems and she thought it had been awkward, and he didn't want anyone else to know about his emotional problems or whatever. But she had already told me this the first time she called me, so it didn't explain anything. The only things she "admitted were that she lied about falling asleep and that it "felt" like he was coming on to her. She said she could see why i was suspicious of her message, but that i was wrong.

Part of me really trusts her and knows that she isn't a sneaky or mean person.

Part of me thinks that would be willing to make a totally blatant lie if it meant me not breaking up with her. She's had a really unstable and unfortunate upbringing, and now that she's an adult, she has a intense need for a stable relationship.

I feel like no rational person could have read the original message and her following excuse, and then believed her. She said things like:

[her] "After sleeping on it, i am regretting what happened, we shouldn't have done that. I didn't explain the situation with my boyfriend to you"

[him] "I don't have any regrets if it makes you feel any better. Friends?"

I plan on confronting her again, although i should have been more prepared for her denying it the first time. I just wasn't expecting it.

Alternatively, i could contact the guy and just ask him. Even though i've never even met him, he might be more honest with me if i was straight-up with him.