Headache after running - Why?

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Well the title pretty much says it all..

I use an elliptical. Im not on any new meds, nothing has changed.. the last several times I have run, I get an intense headache after i stop, and have cooled down. It only lasts about an hour, and I dont take anything for it because it does go away in its own.

So anyone else experience this, or maybe know WHY its happening? Its not going to make me stop, just a little annoying.. and hard to get motivated knowing im GOING to have a headache after..

Thanks in Advance!