guilty or just feeling guilty?

So it was my birthday a little over 2 weeks ago & I went out to celebrate with a couple friends. I would of taken the guy I'm currently talking to, but he's not 21 yet so he couldn't go. To make a long story short, my ex showed up and I did a great job of ignoring him....until the shots hit me and I was WASTED. Like not even a little bit. His best friend happens to be the guy that's been in love with me since high school & whom I tend to avoid because of the awkwardness and because he literally used to stalk me. To get to the point, his friend took my phone, in my attempts to locate my phone I ran into my ex & of course he made me bawl my eyes out that night. His friend shows up with my phone, sees me crying, gets pissed at my ex & makes him leave. Remember I'm wasted. So he tried making me feel better and basically he kisses me & I remember pushing him away and telling him "just take me home"... After that it's all a blurr .. Next thing I remember is us having sex in his car, outside my house. Idk how it started or anything. I felt like shit the next day because I don't do things like that. Especially not if I'm currently talking to someone. My friends made a good point that I'm technically still single and shouldn't fee bad about what happened. They say it's a one night stand that happens to anyone. If so, why do I feel so guilty? 😩