I was trying to put myself in your shoes for a moment to have a bit of compassion and empathy. Firstly, what a difficult realization about the last bf. Leaving more questions than answers for you with such a rush moving in with his new gf and leaving lots of questions if something was going on before the break up, at least for you as you try to recover from that. Quite the sting, no doubt.
Then this new guy starts with expressions of love, then a distance then to run into him one night and to see him with "I assure you she's just a friend."
No wonder this brought out all of that in you. That must have been a most unsettling moment. So sorry that you experienced that.
Probably just cut your losses. And work on recovering from the hurts and pains that you're going through.