What did it mean?

Okay so I initiated a date with a guy whom I had crush on, and we both agree on taking it as a casual kinda date.
The date was casual without any feelings attached.
I personally felt his behaviour to be cold towards me, not very much interactive and when I asked him he said that his nature is like that, after talking for a while, we left and later on i asked about what had happened he explained that his head was aching.
Though this date was not supposed to be a romantic one but i didn't understand what happened here.
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Why he is doing that
What did it mean?
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