Is that a clone because it looks awful small. I would say your problem is one of two things, nute burn or pH issues. Either way there is only one cure. Flush with pH 7 water using 3x the amount of soil your using (3gal pot needs 9 gallons to flush with). You may need to transplant as well. Transplant after flushing. PH issues will cause the plant to show the same signs as nute issues because it creates a "salt lock" that prevents your roots from uptaking proper nutes. Good luck.