Cant quite figure thia one out. help!

hi there. i have been going through so many def guides im going crazy. i think i have this figured out but want make sure. i just turned my lights off for 3 days to switch into flower. my temps at night were getting around the 50'f. its mostly on new growth, and i already know i have pretty good amount of nitrogen ( dark leaves, only sign though) now on the new growth, it started on the fan leaves that were about 4-7 days old so pretty new. ( this is a northern light BUSh about 2.5- 3 feet in height so big tree) now i started getting brown necrotic spots in between vein, then moved to more of a purple the last day of darkness on the very new growth. i usually ph my water to 6.5 or so, im in pro mix myco soil, AGE old organics, grow and bloom. this started when i fed with bloom nutes then a day later went to lights off. my guess is phosphorus due to cold temps. sorry this post is EVERYWHERE. i have yet to medicate today.

oh ya my pot size is a 20 gal.

Ill try and get a picture or two up

pics would help...sounds like it may be Mag deficiency

Its definitely not a mag. I know what that looks like through and through. I've never had this def and I've been at it awhile. Lemme take some pics, the purple/brown spots are turning necrotic and killing the effected leaves. It is only on newer growth which throws me off. It was cold in my room while my lights were off the coldest was about 47-48 and was kinda cold the rest of the time. I had watered the fay before so my rootzone was definitely cold a wet