Are you using tapwater for the changes? If so perhaps it contains chloramine and you're seeing a bloom caused by the residual ammonia left after dechlorination.
since it clears up so quickly I would not consider this a problem.
I know that the source water I use will (at times) seem milky. as I mentioned this is really only tiny bubbles suspended in the water..usually from the hot water side of my tap.
i meant to post here yesterday. Are any of your water parameters going out of whack after a water change? is your substrate getting disturbed when pouring water in.
although maybe it might be a little unsightly i dont know if this would be considered a problem.