If the weeds are things like dandelions, thistles and docks, you haven't removed the roots completely. If they're newly germinated small weeds, this is inevitable - reweed the area, level and sow seed immediately. Weed seed germination is always going to happen if you leave a patch of soil bare for longer than a week or two, and you will probably see new weeds germinating along with the grass seed, but as long as you've got out the roots of the serious weeds, as mentioned in the first sentence, most of these can either be finger pulled as they appear or will disappear when you start cutting the grass.