Am I causing trouble to their relationship?

I have a close friend. We have known for three years and been close for over 1 year. I recently found he has a girlfriend and they have dated for almost two years. I found out because I asked after I saw them together. He hadn't mentioned her.

Well after that things have been weird. He said they are going to breakup after I found out. I got mad at him because he hadn't told me and I found that weird. We have now talked about this and we are friends again. The reason he hadn't told me was because he forgot. He said that he wants me to trust him even though he kept this secret and that I'm important and close to him.

Now I have this feeling that his girlfriend doesn't like me. She seems upset everytime she sees me and doesn't talk to me that much even though I try to be friendly. Everytime I talk to my friend she shows up and interrupts. They are now trying to continue their relationship and make it work. My friend says it's okay to his girlfriend that we are friends but I doubt it because of her behaviour. However my friend seems really happy everytime he sees me.

Am I causing some trouble or am I overthinking this? Should I distance myself from my friend? He is really close friend so I don't want to lose him but I want to respect their relationship.
Am I causing trouble to their relationship?
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