Unless you've used a mucky funnel or something to fill the coolant system, chances are that something that has water and oil in it at the same time is starting to leak.
I'd flush your system, to see if it comes back AND regularly check your oil to see if it starts to look emulsified. Water and oil might be mixing via the head gasket. (Another clue if this happens is that your oil level can seem to go up)
All the best!