Is he cheating with three girls????

My boyfriend and I live together for almost a year now. There have been a couple of insidences where I've caught him sending snapchats to his ex. Pics of our dog, of our vacation for our one year, just weird intimate Hingis like that. This ex, I believe, is the one who broke his heart and his first love. 
And we do this thing where we steal each others phones and "hack" onto their social media posting something that we love them or just a reminder of how much we appreciate each other. Just a simple reminder of "I love you I'm proud of you we're doing life together" kind of  thing. 
Well he's been acting distant lately and he says he's stressed. But he never leaves his phone and is very weirded out whenever I'm near it or ask to see something he's reading me or trying to explain to me. 
Today, was the first in a long time he's left his phone behind when he went to the bathroom. So I took it and was gonna post on his snapchat to be cute. Instead I got curious and anxious because of a dream I had of him cheating and I saw that he had conversations with his ex(the one from above) a girl he used to fuck (whose recently rentered his life) and a girl we ran into at the bar who was supposed to be interested in his friend. All three girls. On one day. Because it was on snapchat, I couldn't see who sent what or what was said which makes me even more scared. 
Ever since I saw this I've had the worst heartache. It feels so heavy and confused. I feel so nauseous and empty.  I feel like I'm falling apart. Like I'm no longer good enough for him. 
I don't know if I'm paranoid. But I think I've seen all that I needed to in order to have a clue of what's going on. 
How do I confront him? Or bring it up? He knows about the dream but I'm afraid he's going to turn it into me not trusting him... What do I do? What do I say?? What do you think about this whole situation?