What does it mean when a guy says he's not sure?

I've been talking to this guy for a few months and we both like each other. We flirt, talk every single day, there's chemistry, similar goals, we don't mind admitting there's an attraction. However I asked him where things were going and he just said he wasn't sure and didn't say anything after that. Did I ask something I shouldn't have? Did I make him feel uncomfortable? Does he even actually like me? How can I fix it if I made a mistake?
What does it mean when a guy says he's not sure?
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