yeah it could be root bound too, you say 3 months old but only 6 days flowering? thats pretty damn long for an indoor grow, there are a couple things i can come up with:
1: if you've been using the same pot for the last 3 months for vegging and only started flowering 6 days ago you've been starving the soil for 3 months and then start to flower with horrible dead soil, no organisms and only the basic nutrients if you only fed it basic nutrients which works perfectly fine for a 8 week flower but higher than that you should really consider buying micro organism encreasing stuff like molasses and all those extra soil stuff like that stuff that gets rid of dead roots and turns it into good organisms, im really high so i have absolutely no idea what they call them.
2: your soil will get really dense over time cause of the constant watering and drying, unless a good soil mixture of coco or perlite or anything else that encreases air in the soil this wont really happen but it does happen faster when growing for a longer time.
3: If you dont have holes under your pot it will grow roots untill they all strangle eachother and die at once, i'd imagine it looking a little bit like this if that happened. also this most of the time only occurs when you grow for a longer time in a smaller pot.
the only thing i can come up with is that you've vegged it for too long in a too small pot or not enough nutrients, i dont know anything about serious nutrient deficiencies as i dont even know how to type to word
I hope i didnt insult you with anything i just randomly assume you have 0 knowledge about growing so its a little easier for me to type the story, if you have considered all these things and im just talking shit then so be it.
Good Luck & Peace!