I [21M] found nudes of my girlfriend [22F] that she did not send me

Pretty self explanatory title but I'll give some background. I have been dating my girlfriend for over a year. On our 6 month anniversary, she ignored me all day and I caught her texting her ex right in front of me while I was trying to talk to her and I had paid to take us both on a vacation that she asked for. We talked about it and moved on. Around this time, I was also given an STD by her, which she swore she must've contracted 6 months before me, even though we were sexually active that entire time and I showed no symptoms. I say all of this to say that there had been some existing trust issues before the incident I am about to describe.
We are very open about using each other's phones and it has never been an issue, as I felt like I had nothing to hide and she often asks me to do things on her phone for her while she's busy with other stuff. When I opened it, it was on her Snapchat, looking at the "my eyes only" section. I asked why this was and she said she didn't know, she hadn't been on it. I asked her if she promised me and she said yes. I didn't truly trust her, so I asked her to unlock it. She did, and I found an explicit video of herself that was taken while we were dating, about a month before I noticed STD symptoms. It was nothing along the lines of anything she had ever sent me, (We barely sent nudes back and forth as this isn't something I ever asked of her or expected) and I asked her "did you make a video of you doing ___?" she said she may have made it for someone, I'll call him Joe, because it used to be their "thing" but it was before we started dating. I asked again and she said the same thing. I then showed her the video I was looking at and she said she didn't remember making it, and that it must have been for me but she ultimately decided against sending it to me.
I immediately knew this was not true and asked her why she would lie to me about it, as what was in the video was something she had done before that had upset me, and she knew I particularly did not like. She insisted it was for me and we argued for hours before she finally admitted it wasn't and she had only told me this to get me to not worry about it. I never raised my voice or got angry with her, more than anything I was sad and hurt and worried.
Fast forward to later on in the week and I had time to give it more thought. I questioned her on it again, explaining whatever the truth was would be okay, I just needed the peace of mind of knowing what really happened. I had looked at my calendar for the day the video was filmed, and realized that it was taken right after I had gone to work but had spent the entire day before that with her. She swore to me that she filmed it for herself just to see what she looked like and sounded like (there was lots of noise in the video) but the part I can't understand is why she would do that if it was never something we did together, and it was something she'd done with someone in her past. Not to mention, she said she felt skinny and wanted to see how she looked, but I have never heard of anyone filming the kind of thing she did that was extremely explicit for that. Most girls I know take pictures in the mirror of their stomachs or whatever just to feel good, which I understand.
The other part that got to me was why didn't she just say that from the start if it was the case? She lied about the video's existence, claimed she never remembered doing it and never went back to watch it, yet filmed it, deleted it, put it in my eyes only, and then lied to my face about her motives for everything.
It has been a point of contention for us the past month or so as we struggle to decide if we can still be together and if I can ever trust her again. Should I break up with her and throw away a whole year with a girl I care deeply about on a "gut feeling" that I have been cheated on?
TL:DR Found girlfriend's nudes, looks like she may have sent them to someone else while we were dating, lied to me multiple times. Not sure if I should break up with her over it.


Source.