(19/f) should I believe my boyfriend (22/m) or a random girl??

Okay, first off a little bit of back ground information. My boyfriend and I work together, so we met in August and started dating in October. We've had this guy, Tom, trying to ruin our relationship for a while, but I thought he finally gave up, and we haven't worried about him for a while.

So, yesterday while at work, I got a message from some random girl who claimed that my boyfriend had been messaging her on a dating site, and that she had screenshots and proof. She then talked about how they had been texting nonstop, and when she looked him up on Facebook, she found me and asked him about me, and he denied knowing me.

Of course I asked him, and he got really defensive and started to get beyond upset, swearing on his grandma's grave that he would never do that to me. He's never done anything wrong to me for me not to trust him, but with her saying she had screen shots, I wondered. So I asked her to send me the screenshots, and she said she didn't have them anymore, so I asked her to send me the texts, and then she said they never texted, even though one of her first messages said they did.

So really she couldn't prove anything, but she "promised" she wasn't lying. Three of my friends, my mom, and my sister all think it was a set up from Tom, because they don't see my boyfriend doing something like that. He posts about me on his Facebook all the time, so it's kind of hard to deny me. I just don't know what to think. As far as I know he's never cheated on anyone, and I've seen his messages and snapchats, and there's never any girls, so it could be a total lie trying to break us up, but then again it could also be the truth and he could have deleted everything and acted innocent.

I just really don't know what to do, because everyone that personally knows him says that they can tell he truly cares for me and wouldn't do something like that, but I've had awful experiences in the past and know not to let my guard down. Please, tell me what you would do!! Thank you!