Sick Plant

Hey, so my guinea pigs plant is about day 75 into flowering its 17.5" tall got 3 42w CFLs on her and she grew bout an inch a day than slowed down..stopped and its been a lil a week since it started i know its not the pot its atleast a 3 gal pot , all the fan leaves starting from the bottom workn its way up are getting brown spots all over them than turning a pale yellow turns crispy and dies off, its happend to about 3 sets of fan leaves now and i dont know what to do, ive checked the ph said it was between bout 6.5 and 7 my nitrogen was depleted potash and phos were good so i put some 24-8-16 MG into her,almost all the new growth tips are burnt but i dont see how it could be fert burn because when i switched to flowering i also switched my soil from MG potting mix to some organic soil from dollar general, plz help, i dont want my first grow to go under when ive come this far