Did my friend go behind my back to romantically date my ex-boyfriend?

I don't know what to do. My chest aches everyday and I feel so weak. It's (barely) 3 weeks after my ex-boyfriend dumped me and I thought I was over him, recently through (somewhat) unreliable gossip I found out that they have been, "dating". I found out by a "friend" saying she heard people saw them kissing in the hallway. I was distraught and asked my ex. He said yes and I freaked inside. Apparently they had been going out on "dates" for the past 2 weeks. But this is where it gets tricky. I asked my friend if they were dating and she said no. Then I asked her if they had been going places alone and she said yes. So she admitted they had been going on "dates" as friends to find out more about each other as friends then I said dating is romantic, that sounds like hanging out and she said that they were just hanging out then. Supposedly. I don't know if it's truth or lies. She does have a crush on him and she is really pretty and has sex appeal so I know my ex would most definitely date her if she asked him. She told me though that if I don't want them dating, she won't date him. They're both moving anyway. My ex to north Carolina (4 hours away) and my friend to a college in middle TN. (We live in east TN) So I told her I couldn't condone them dating and said she'd take it into consideration. Another girl asked my friend if she was dating my ex and she said my friend spun around and said, "Shh, it's a secret." I asked my ex what kind of dating they were doing and he said they were getting to know each other better as friends. Now my ex won't even respond to me over texts, my friend won't respond either. Before I knew about the "dates" she had bought his prom ticket for him after she bought hers. My friend has never lied to me and I've trusted her so much but now I don't know what to do. It hurts horribly when I think about them dating behind my back and lying to my face. I don't know who I should turn to. Help? Is she dating him behind my back after she called me her "best friend"?