If it only happens after you use it then it is a leaking drain connection. If it goes on all the time then it is one of the supply lines to the tub. If there is no way to look as in under the tub then you will have to remove whatever covers up the faucets on the back side. If that exposure is on the outside of the house or someplace that has a lot of stuff on it then it get complicated.
Bill Morgan, October 2010