He might be cheating...

Hello. Been with a guy for about 6 months now (both early twenties) and it's been going really well, we see each other loads and go for meals/walks/watch films and study together. He's so lovely to me and really does so much for me. However, for the past few weeks or so I've had a feeling things aren't right - his phone has been off sometimes and he says it's because it keeps turning itself off but whenever I've been with him it's been working fine for days at a time. Also, I (stupidly) had a look at stuff he had written on a forum and he wrote something a few months back (in a round about way) about having gone home with a girl on a night out. I had a nose at my phone and realised he called me that night at 5am and was texting me all morning the next morning. So I don't know if he was maybe lying on the forum, as sometimes happens? Feeling the need to confront him but not sure whether to mention the forum, or just in general say that I've had a funny feeling for a few weeks. Thoughts? Would appreciate some advice :)

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