I need some relationship advice. Is my girlfriend cheating on me?

So i feel like my girlfriend might be cheating on me with my friend because her behavior has changed and she seems happier around him, she has now started to call him a kinda weird nickname she has never in the 2.5 years of them knowing each other called him a nickname, and i feel like a third wheel when im with them and im able to walk off for about 20 ish minutes before they realize that i have left

I have more details if you want them

I need some relationship advice. Is my girlfriend cheating on me?
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