Do you think he's cheating on me?

My SO used to walk me home daily. He has a girl best friend who I know has a crush on him. She talks about him nonstop to everyone and talks bad about me. She says they hang out every other day and he says they hang out once or twice a month. They text nonstop and he won't let me see their messages, And now skips out on time with me without giving me a reason. He also ignores me completely around her and has more jokes with her. Do you think he may be cheating on me?