What are the signs that someone is cheating in an Long Distant Relationship?

Me and my girlfriend have been in a relationship for almost 3 years, but now i think she's cheating on me. She seems to become more distant towards me, gets angry easily, calls are getting low, seems to always mention a guy friend, no more phone sex. I feel so depressed right now guys. Someone tell me something.
What are the signs that someone is cheating in an Long Distant Relationship?
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