Ok this one is a little complicated but I've tried my best to explain it. 

I've been seeing this girl for a couple months now so I know her pretty well but not completely. Basically, my girlfriend at this point but we haven't made it official. 

I'm back home visiting my family so the other day, I sent her a picture of me and my cousins hangin out, expecting her to say something like "aw that's a cute pic". But she proceeds to nitpick something about the picture, the details of which are not important to the story. I was a bit annoyed saying these little details she's nitpicking on aren't important, just focus on me and my cousins. This wasn't such a big deal to me, these things happen. 

However, she said she did say it was a cute pic. So I scrolled up thinking I missed her comment but I saw no such message. I took a screenshot of her non-message and sent it to her. I was expecting her to send a screenshot as well with her message but she didn't. So I asked her to send a screenshot of her message cause I just had to see it for myself. I know it's not really trusting of me to ask that instead of just taking her word for it but that's what I did. She ignores it completely and keeps on with the convo. I asked her again about the screenshot and this time, she seems or acts confused. I told her I asked for a screenshot and she said that she didn't get that message and sent me a screenshot with that message missing. At this point, alarm bells are going off in my head. Is it possible that she deleted the message I sent asking for the screenshot, took a screenshot after that and sent it to me? How is it possible that two very convenient crucial messages didn't go through? One supposedly that she sent that didn't go through (her saying it was a cute pic) and another one that I sent asking for the screenshot. By the way, the message that I sent asking for the screenshot is showing up as 'Seen' so she MUST have seen it right? 

I called her out on this and she was shocked and completely plead ignorance. She said her phone's been acting up and that I should believe her over a phone. I should also say that I'm overseas and my internet connection is not very fast. I pressed her a bit more about sending me a screenshot of her initial message and she finally sent it to me. The message that she said she sent (cute pic) wasn't in it. And then she said that's probably because it didn't go through or that maybe she typed it out and didn't actually press send, which sounded like complete bs to me. So after a back and forth of asking her to admit it and her not admitting it and sticking to her story, we're pretty much at a stalemate and still fighting about it. I know it sounds like I'm the one being petty here but the evidence against her is there. I don't put anything past anybody and I feel like people are capable of anything. I had an ex, for example, who would log into my fb account and look at my messages. It might not be fair to compare this girl to that ex but I can't help but operate on my history. Part of me wants to believe her but there is clearly another much bigger part that simply can't let this go. AITA?