If it's a few leaves, it may not be serious. It sounds very much like iron chlorosis which can be caused by a variety of things, but usually because the soil pH is too high. One approach would be to apply a chelated iron fertilizer. This would be faster than applying sulfur to the soil since it takes one or more growing seasons for it to fully react and lower the pH. However, since you just fertilized and the new growth looks healthy, I would wait on doing this. To make sure chlorosis is the problem, send photos to this email address.