I 19 f am with my gf 26f. She has this “ex”. She doesn’t call her an ex but they were together. Her ex just became single and wants to be friends with my gf now. I told my gf I want to hangout with them when they hangout together. Her ex wanted to make plans to go to the beach today. I wanted to go to. So she told her but she ignored it. I woke up early than my gf this morning and I went to the bathroom and I wanted to play games but my phone was dead so I went on her phone and I ended up going through their text messages. And I saw that they hung out together 2 weeks ago that I didn’t know about. We also haven’t had sex in a few weeks. I asked my gf when was the last time she saw her ex and she said like a few months ago when we were all together. So she lied to me about it. And I think she cheating. I don’t know if I’m reaching for something that isn’t there for if I have an actual reason to believe she is cheating. Can some please give me advice on what to do.