Hi Lucia,
One possible explanation for feeling an urge to breathe or even contractions in colder water than you are used to (25C is pretty crazy, though! I don't think I've ever gone freediving in water that warm
) is that your blood does not hold C02 in solution as well when it's cold as when warm. So the sudden feeling of an urge to breathe is caused by the drop in temperature and the release of extra C02 (probably carbonic acid). This I guess is the reason for a lot of drownings in cold water. The shock of cold water causes the person to gasp (burning lungs) and then they panic and swallow water or whatever.
If I'm doing a no suit dive in anything colder than 14C, I'll hyperventilate for 30 seconds, really hard, before getting wet. Then I just usually breathe passively for 1-2 minutes until I'm relaxed. It makes a big difference, since the hyperventilation blows off some of the C02 that comes out of the solution, bringing me back to my usual equilibrium.
Hope that works for you.
Pete
