Am I overreacting and being insecure? F21

I recently got into an argument with my boyfriend (M22) because of what I saw on his phone. He was screensharing his phone and I think he forgot that he still was so he was going in and out of apps. The two apps that caught my eye was his photo album and snapchat.

On his photo album I saw pictures of a girl which he has since explained was his ex from three years ago and that he must have missed it when he was deleting her from his photos which I think is pretty valid because I remember having a couple of pics of my ex that I had missed whenever I was deleting pics of him. I will admit I did overthink badly about this, but he has never really given me a reason to think he was still hung up on his ex so I think he simply just missed it when he was deleting her too.

Also I want to add that I am totally okay with him having girl friends. But earlier in the day we were talking about his high school/college friends and he was telling me memories about them. I think eventually I teased him that he must still stay in contact with a bunch of his girl friends. He told me that he actually didn't and that he doesn't have any girl friends anymore and hasn't talked to any girls in months. So hours later when he's screensharing and goes on snap, I see his recent chats were from girls. There was one that caught my eye though that said they were #1 best friends which I think would indicate that they talk pretty frequently. I don't use snapchat much but I assume that's what it means right? anyways the whole reason I got upset is because he lied. Whenever I confronted him I tried my best to not assume the worst and tried to come to him calmly and asked who the girl was. I don't know if he was acting stupid but he kept telling me "who??" He just kept asking who I was talking about because his recent chats were with his sisters apparently and his best friend and mom (?) and he kept insisting that he didn't know what girl I was talking about until I said "i have proof, I have a screenshot" which is when he finally said the girls name and asked if i was talking about her. Immediately he said she lived in a different country and that apparently she wasn't even a real girl. He told me she was just someone he used to play with and they had started talking two weeks ago.

Anyways, I feel that this isn't a big deal and maybe even childish but I feel iffy that he felt the need to lie and not say her name until I said I had proof. I also wanna add that he is the type of person to rarely text people and is a slow responder. Even a couple days ago, i had gotten busy with work and school so I was not able to text him at all until the very next day which made him upset and he said "you and my best friend (male) are the only people I ever text so imagine how I felt whenever I didn't get a message from you"

I don't know if i should be suspicious or not if there's possibly something going on or if it's even weird and i'm just overthinking?? I don't have proof that he is doing anything bad nor has he ever really given me a reason to not trust him. I want to so badly add that girl from snapchat and ask her about her friendship with him but I think that would be crossing the line and that it would be toxic of me to do. He's provided me a few details about her so I just wanna see if it's true. Am I being insecure and should just try to move on? I've tried asking him for reassurance but he doesn't feel the need to do so because he feels that he hasn't done anything wrong but I feel so paranoid about this. Something is telling me that there's something more. I've tried talking to him about it but he seems to get annoyed and doesn't know what to do about how I feel. So maybe I should try to move on? I don't know. I think I just wanna know if this is all silly and i should just move on from it.

Edit: we have been long distance for almost a year now and this is my first LDR which is why i feel a lil more uneasy about this