The other night I'm talking to my gf on the phone and she tells me she needs to go pick up her mom because her car broke down. She told me she would call me on her way home. Well an hour went by so I texted her to see if she needed help or anything. No replay. Then another 30 minute go by and I call her and get no answer. So she finally picks up and says her phone was in her car and she was in her moms house. She ALWAYS has her phone on her though. I really trust her but this has me thinking.
This.... ^^^^ Heck, she was gone an hour and a half? this relationship won't work if you feel that insecure.
I'd look for a pattern for a longer period of time before coming to this conclusion. But I agree with the others here, that you have to trust your girl.
If you think she cheated, dump her. Personally, I think you sound very insecure and needy if you can't even go an hour without talking to her. She probably just wanted to talk to her mom for a bit without you butting in. And of course you jump to the conclusion that she MUST be fucking someone else if she isn't at your beck and call. On second thought, ask her if she cheated so she can dump you.