It is a very common weed. The seeds may have been in your soil or in the load of dirt.  From your description it, thankfully, they were in only one spot.  The seeds could have blown and only contaminated the top of a plié of dirt, for instance. They are not hard to get rid of and an application of broad leaf weed killer for lawns or a weed and feed product for lawns will get rid of them. Wait until your lawn has established itself before applying it. Use a product for new lawns or use it after the lawn has been mowed 4 times. Follow the label carefully, read it before you buy to make sure the product is the right one.