My boyfriend looks at pictures of other girls?

My boyfriend has always said to me that since we have been together, he hasnt been even thinking about other girls, and definitely not masturbating over other girls. But the other day he was sitting next to me and he pressed a on on his phone (which makes whatever show up that he was just looking at), and there was a "y" picture of a woman in underwear. I think he saw that i saw it and then he quickly hid his phone. After that i got a bit paranoid and looked at his history on his phone, And there were lots of pictures like that. I then confronted him about it, he seemed very uncomfortable and told me i was overreacting. I just feel kind of sh*t. Like he would rather look at these girls. The thing is we are together so much i dont know when he is even looking at these pictures:s. What should i do about this. It is really upsetting me.