I Feel Bad Now.....

So, I work the night shift at a gas station and occasionally creepy people come in. Tonight a random guy came in. I was making myself a sandwich and he was looking at something behind me. He randomly turned around and said "if I proposed to you, would you say yes?" Now, I have a boyfriend who I am madly in love with so I just kinda liked at the guy like he was crazy. I didn't realize at the time that he was seriously flirting with me. I thought he was joking. So, my response was "Nah. I'd have to get to know you first." I was playing along with his joke. He said "would that be possible?" And, again, I jokingly said "eh, maybe." After I said it, I realized that it all sounded like I was flirting with him and he made it clear that he thought I was flirting with him too. I feel really bad now :/ I love my boyfriend and i'd never cheat on him. I didn't mean for any of what I said to be flirty. I have no interest in any other guys. I just feel so bad :(