Its unlikey you have nitrogen toxicity, unless you added nitrogen yourself, it wont be in the grow media in toxic levels, especially if your plant grew a foot in said growing media, cannabis is a big feeder compared to most general plants, sure your growing media may be more ideal for veg rather than flower in that case you can repot, cutting your potting mix with 50% pumice sand or perlite, without disturbing the roots in repot, i think you may have over potted it in to a too large pot initially and possibly over watered it, you should let the potting mix dry out between watering cycles, but im stabbing in the dark with no photo and little info.
add a photo and ill let you know the real problem its not your MG soil/potting mix, now you must remember that we all come from different parts of the world and im assuming MG is a potting mix brand you have in your country. my guess MG stands for something like magic grow, no idea as we dont have that brand here