Anyone know what it could cause the yellowing leav...

It's not nitrogen!! Not at this stage of the plant's life... The leaves look like they're the very first leaves that grew and their yellowing/dying off is normal if they're very close to the soil. At this stage, you're wanting to cut back on the nitrogen and start increasing the potassium and phosphorus because all the plant is wanting to do is to work on the buds and get them fat - and they do this with the P and K... You don't have any information on what nutes you're using but please - do NOT increase the nitrogen - that will only lead to lockout of calcium/magnesium/potassium/phosphorus! Those leaves are best just trimmed off and forgotten... And btw, to get rid of your obvious fungus gnat problem, use some Mosquito Bits which will kill off the adults/eggs/larvae ... just soak the granules as directed and use the water for your plant. Fungus gnats live in very moist soil and since you have a problem with them, it says to me that you are keeping the soil much too wet. Cannabis needs a wet/dry cycle to grow properly... make sure your soil is completely dry before giving them anything more.... lift up your pot to see if there's any weight to it or not - if not, water/feed... if you detect any weight in it, skip watering/feeding until you don't. Good luck...