Regretting sex

Have to get this off my chest. Last night this guy I’ve been talking to wanted to come over. I told him he could come hang but I didn’t want to do anything. He comes over and starts making advances and we end up having sex. As soon as it began I had a pit in my stomach and instant regret happened but I didn’t say no or stop him. After it happened I still have a pit in my stomach and I know I’m being dramatic but I wish this feeling of panic would go away. It wasn’t my first time or anything but this is the first time I’ve ever regretted sex with someone and I don’t know how to cope with that