did you check for mites?? look at the top leaves, the lil tiny white dots... you should check and see if there are any, i hope i'm wrong... i got a clone from a club and it had some mites on the bottom leaves, spray with oil/soap/water solution and it will be good.
i think its a nutrient burn cuzzz i have chopped up garlic around it and i think thats keeping bugs and shit away.. but i always look for wierd stuff on it everyday