I've been "swimming" once in a while for years, while on vacation for example, and never had a problem. Recently I've been learning the crawl (freestyle). I've finally got myself up to 20 meters or so without running out of breath. But now, after swimming the crawl for a while, I feel sick. I come out of the water and lie down, and things spin (as if I've drunk too much), and I feel a little like throwing up.
I'm pretty active. I run and things like that. Is the nausea a sign that I'm doing something wrong, or is it just part of getting acclimated to this stroke?