I had a very similar problem and it was condensation. I was not getting close to operating temp. I had plumbed my exhaust wrong and could just get it barely above 100F. Changed it (thanks to the awesome help of some SW members) and now run around 140-150F and problem solved.
It is probably condensation due to the cold water temperature this time of year. Keep an eye on it and see if it goes away when the water gets warmer.
Check to see that the flappers in your Y pipe are still there - If they are gone they are probably blocking the exit at the bottom of the Y - This will cause the water to back up inside the exhaust and that will cause reversion.