Hey Growmie,
It does seem that you are in the beginning stages of a deficiency, specifically a nitrogen deficiency. I went through your diary and I didn't notice what your ph was, if it's not in range this could be causing the problem. If your ph is off this going to be a lockout of nitrogen meaning the plant is not intaking any nitrogen. CHECK YOUR PH !!!
Also, congrats on this being your first diary, I wish you nothing but the best in your grow journey.