he hides his phone -- is it me or him?

my bf hides his phone from me now. I was sitting with him once and I noticed on his instagram that one of the top people on his viewer list for instagram stories is this one girl. I know that the top people on your viewers for stories are people that you interact with regularly. I asked him who this girl was and he said it is his coworker that he doesnt hang out with. I asked him more questions and he ended up telling me that he used to hang out with an old drinking group and she would mutually be there. he says that he doesnt hang out with this group and said he would never cheat on me.

ever since that talk he has been hiding his phone from me.

should i be concerned? or was it wrong for me to ask and now he doesnt feel comfortable around me with his phone.