Would you consider this cheating

So recently I came across on my boyfriends Instagram,Facebook and Twitter! That One of his ex girlfriend that has been already blocked on each thing 3 times. Every time I find her unblocked he uses the excuse that his friend did it or well you have your coworkers added on everything. Before he left for a week of work he was looking at a picture of me and he sent me a screenshot and behind the picture of me I could see what apps he recently was on Facebook, Twitter and Instagram. The Instagram showed a conversation with someone and I asked him. He 100% denied and told me this "Because I was going through deleting!!! Jesus (my name) Nothing was going on, everything is ok, and I love you. I was just going through it it to delete it" I then had said I don't trust that bitch and he comes back with "You sure it that bitch or you sure it's me?" And I said I trust you but not her. The conversation went on and we dropped it. But me being a worry wart I was snooping through his Facebook and saw she was unblocked but not added yet, okay so I move to the next thing I know Twitter go on there see they are following eachother. So I go ahead and block her on there and Facebook. Yet I didn't know his Instagram account login I knew his passwords so I tried one. I got in, I saw there was a pending message to be read saying "I know you won't see this right away, but do you ever feel like we weren't given a fair shot?" Which instantly made me mad and hurt because it was from his ex. I scroll up to see they had been talking since the 30th of March ! So I ready five messages and freak out! Wasn't sure what to think so I threw my phone at my sister to ready and she had read them out loud saying what they had been talking about even including the emojis they had used back and forth! So me hurt and confused I have to wait till Friday this week for him to talk to me and respond to her. Everyone thinks I should stay low and act like I don't know there talking till he responds to her message and when he says something back to her let him know that I knew they were talking all along..... Opinions .... PLEASE!