Why do I get a headache during (and after) swimming?

It could just be that you're swimming to hard and to fast for your fitness level. This happened to me as well when I first started out swimming. I'd say try slowing down a bit, rest more often and not every time you feel the headache coming on, and do not go as far. Swimming endurance is something that has to be built up over time. So take it slow and see if that helps =). Also, unless you're really sure you're doing everything correct. I suggest taking a class or getting a coach or someone who knows about swimming to come and watch you swim, at least once. When I teach swimming lessons my kids (and adults) get headaches from bad technique on their strokes and from breathing wrong all the time! So I suggest that, and maybe it'll help.