We installed a new lawn a while ago. Did everything proper, removed old dead grass, tilled, added top soil, seed, starter fert, watered. Lawn looks great last few weeks. Has been cut 4 or 5 times. About a week and a half after applying scotts turf builder fert, now there is several yellowing spots in the lawn. I dont think over-fertilizing did it, and it has been watered plenty. Any indications of what this might be? Lawn is mostly weed free, and no grubs were noticed when installing lawn.