Ex and best friend

My ex has started dating my best friend Nd I just found out about it a day after we were on FaceTime being freaky when I had texted him and told him I can tell he got a new girl and I asked who, she had texted my phone and told me her. Isn’t the first time she has dated one of my exes. And the crazy thing is me and my ex had been talking to each other for a whole month after the break up and had been trying to get back together. Then a day later he’s dating my best friend?!! What should I do? Could this be revenge because he set me up to see if I was loyal and used an older ex? I mean... deep down it seems like he still loves me even though I have made a couple of mistakes with him because we argue and make up and out of all people my best friend?!! He has to still care about me