Breathing

In my 13 years as a Master swimmer I think I have a pretty good technique and speed, although recently (in the last month sometime) I was reading a thread where someone mentioned about breathing. She/he said that breathing while swimming had to be very much like breathing normally out of the water. All the time since I started to swim I was used to hold my breath until a second before breathing again, so I let the air out in a single push. Last night, for some reason I decided to try letting the air off the all period that my head was under water. After several hundred meters everything was well and I though I did not felt very tired keeping my normal speed, but then I started to have a headache and it lasted me for an hour after finishing my training and must of the times I do not have headaches. Anyone can tell me if this is normal for the first time of doing it, or the technique for breathing should be different? Alex.;)