Idk what to do...

My girlfriend and I have been dating for around two years total in April. We're both fairly young and our relationship has been going really well, there's been ups and downs obviously but for the most part it's been great.

She's my first girlfriend but I'm not hers which has been fine but recently she's become really distant and stand off ish. We went out together with my dad and a few of he's friends the other night and I got sick on the way back. She was really caring and made me feel a lot better but for a little while she was on her phone (while I was vomitting so I'm assuming that she didn't think I was looking but I did) and was messaging someone on Snapchat which I usually would have thought was fine but their name was in her snap as 'pretty boy' I didn't think much of it until she sent a photo of herself and he responded "My beautiful girl" to which she sent back love hearts and continued talking. I know that she's bi so I knew there was a possibility this wasn't platonic.

I don't really know what to do because I never thought she was the sort of person to be unfaithful and I'm scared to confront her about it. I told one of my friends and that's all. I don't know what to do though because I really love her and idk what I'd do if it turned out that she actually was cheating.