If always after a water change, then this could also indicate that his gills are irritated from the newly added water. Yawning could be an attempt to flush his gills. Could have something to do with any additive that you are OR are not adding. If you notice that after a few hours he has stopped, then its probely because what ever was in the water has dissapated by then.
you shouldnt really notice any change in activity pattern in your fish after a water change if you are doing enough and correctly(temp the same, water treated ,ect..)
I highly doubt its parasites.