Grub damage usually isn't apparent until late summer, so it probably isn't that.
There are many, many problems with lawns that could be yellow patches. It's difficult to diagnose them unless we know the type of grass, your part of the country, and other specifics. However, you might consider calling a good-quality lawn service to diagnose the problem for you. Many do this for free (call around) in hopes that you will then hire them to solve the problem. But even then, it can be hard for a lawn professional to diagnose because again, so many lawn problems look very similar. Still, a professional consultation is your best bet. Another option is to dig up a sample and take it to your local office of the Cooperative Extension Service (check the phone book -- it might be listed under Extension Service or Agriculture Extension). Call to see how to collect a sample. If no one at the office can diagnose it, they can send it to a pathology lab.