It happened to me as well! When I googled it I read to just rinse the tubes with water like a hose, and to hang them over something so that the water drains out (alternately, if you have enough space and are in a hurry, whip them around your head like a lasso). Another suggestion was to use rubbing alcohol. Either way, before the next time you pump, run your pump for a few seconds before you start to pump, it'll pull air through.
As long as the milk doesn't get down the tubes and into the motor, it's no big deal. And if you have an Ameda or Hygeia, then you especially don't have to worry - medela is the only one with an open system.
Re: Milk in my pump tubes....