how do I find out if my girlfriend lost interest in me and if she's cheating?

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Me and my girlfriend goes to different high schools And I think she might be cheating on me

When school first started me and my current girlfriend just started talking again after our break up and she begged me to come to her school, but in the middle of the year she told me that there was another boy and that she was stuck In a sticky situation on who to choose so I immediately asked her out the next day and she said yes. But I think she still might like that boy or maybe a different boy because she admitted that she not sure if she wants me to go to her school anymore and she acts weird lately

She told me she couldn't see me after school because she has to go straight home but I recently found out that she goes to this park after school almost every day and the park is very far from her house and school. Its about a 45 minute to an hour walk

Also when we talk on the phone and her friends comes around she quickly hangs up the phone on me

she takes a while to text me back, and I noticed that when she doesn't respond to me, she's on Facebook

Lastly she gets a attitude with every little thing I do. For example I can make a joke and she'll get mad and wont laugh, but if someone else said the same exact joke she would laugh and talk about the joke day.

I dont know what to do or think anymore

can someone please help me out?