Why would he suddenly become cold?

Started dating with a guy and it was going really well. we were both looking for something serious and we were going out 3-4 times a week. He generally expressed interest in any way and communicated regularly expressing how much he liked me. The day after our first time, he also get in contact with and I thought everything was going great. However the second day he became cold all of a sudden and he left my last message on read. I literally can't figure out what has changed. if it was about the sex he would disappear the next day but he didn't. Should I leave him some space or demand an explanation?

Why would he suddenly become cold?
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