If it was a big enough starter, it may very well have finished that quick, that's the point of a starter... to get a very quick fermentation.
Or, it could still be fermenting, but the beer is absorbing most of the CO2 so your airlock is tricking you into thinking it's not fermenting.
Or, if your mash temp was too high, you don't have as many fermentable sugars and the yeast finished off everything.
Or, the temperature could have dropped significantly, slowing the yeast down.
There could be a thousand things.
I would wait a full week and then do a hydrometer reading, and go from there.
EDIT* : Now that I think about it, I made a stout with WL Irish Ale and I remember it fermenting normally for about 3 days then 'stopping', I waited it out and it was finished as planned. And I didn't make a starter. Had I made a starter, I may very well have had the same results you are describing.