Did it start with younger leaves? That'd probably be a calcium problem. I have no experience growing, but know how to use google like a mofo. From what I've read, a calcium problem will start with your younger leaves.
Too much P, and not enough N is another solution to your problem I've found via google.
...take what advice I give with a grain of salt and shot of tequila, because all I know is what I see on google.