I think I caught my girlfriend cheating. What do I do?

I 18M called my girlfriend, 19F and for some reason I got the glitch where it answers a few seconds before it answers on my end. I heard a guy say (my girls name), who is (my name)? I asked her on the phone who was talking and she said the TV. I clarified saying I heard both of our names. She dodged the question and I asked again. She just answered with “yeah that’s weird” we have been dating for a 4 months. I know it’s not long but we’re really close at this point and spend almost every day together. At this point what do I even do? I have had no reason not to trust her until now. And now I’m having doubts. The fact that I heard both of our names is really making me uneasy.

TLDR - I think my girl is cheating because I heard another guy on the phone asking who I am in the background.

Edit: I’m going to work things out in the next few days. I didn’t think anyone would be invested in this. 2-3 comments, tops. I really appreciate everyone’s response and input. I’ll try my best to respond to everyone. Thanks everyone for your concern and best wishes.

Edit again: for some more context and repeated questions I’ll include a little more info.

Yes we are supposed to be exclusive. I do not think I am the one she is cheating with. I haven’t noticed much different behavior. We spend most of the day and usually the night together.

This took me by surprise. She’s always been open with me about everything. I could almost instantly tell something was off, even if I ignored her mishap before she thought she answered or whatever the phone did.

Edit 3: I really appreciate everyone reaching out with their advice. Hence the subreddits purpose. I wish I could reply to everyone but time just won’t allow me to. As of right now I’m not speaking to her. Not going to say or do anything. I don’t condone lying or cheating and I’m pretty upset to be honest. I can’t confront her face to face like many of you have recommended, because I won’t see her for a while. I don’t think she’s worth my time and I won’t be continuing this relationship. If anything more comes of this, I can come back and update you guys; it seems as though many of you have become pretty invested in this story. I honestly didn’t think anyone would care and looking at the post, I’ve received well over 2 million views, hoping she isn’t one of them (I don’t think she has Reddit anyways). I don’t want to be walked on and I don’t want future hurt. I’ve decided Im going to take the advice of those older and more experienced than I am and break away. I’m sure there are other women out there that will treat me better. Thank you everyone for your input and advice.