Why am I like this ?

So I’ve been with my bf for almost 4 months now and so far he is the one who initiates dates and texts. I don’t really have any doubts about his interest in me , however whenever he is on his phone around me , which doesn’t happen often I get this weird feeling that he might be talking to someone else.
He doesn’t hide his phone, when we are in bed he puts his phone on the bedside table next to me . But my weird mind then thinks yeah what if he put his phone on airplane mode . Like I am trying to find something that isn’t there .
This morning I was eating breakfast and his phone was right next to me but again maybe the wi fi wasn’t on .
I just can’t stop myself from feeling really insecure when he does use his phone. Another thing I’ve noticed is when he reads the news on his phone etc as soon as I pick up my phone he stops being on his phone.
How can I just be chilled and trust my man that he doesn’t do anything behind my back ?