N Toxic or just flowering ?

High all,
I had a recent grow of 8 plants. They were all suppose to be Auto-Flowers but one turned out to be a photo. This is a Seed Stockers Big Bud plant. Anyways, it vegged until the Auto's all finished (about 10 weeks). Well it's been in flower right around 3 weeks now. Lots of Flower tops but they haven't really turned into buds yet. Well, about a week ago I started getting yellow leaves toward the center of the plant. Now there's a few closer to the top & some at the bottom also. None of the leaves have burnt tips so I'm thinking it's NOT a N toxicity. It's just that none of the plants I've grown in the past have ever yellowed this soon into flower. I'm wondering if pH is causing this or maybe it's root bound & that's the cause. I only up-potted from a 2 gal. to a 3 gal. pot right before putting it on 12 / 12. Looking at it now I should have used a 5 gal. pot as the plant looks too big for the pot.
Well, too late to up-pot again since it's 3 weeks into flower already.
I'm wondering if it's OK to flush a plant at this point in the game just to ensure the pH isn't the problem. I know you'd like pics but my batteries died so have to buy new ones before I can post pics.
None the less, the leaves look perfectly healthy other than them turning yellow so soon. I did notice that the other 7 Auto's yellowed faster than photo's but still not as fast as this one is. I had some buds on the Auto's before they yellowed. I'm not sure if this is something I should worry about or not. The process seems normal other than it's happening faster than usual.
What's your thoughts on this & is it OK to flush at this point ?

What about an N deficiency?

An N deficiency would be normal during flowering so you're not suppose to add N during this time. I'm sure there's enough N in the nutes I feed once per week. I only water & feed every other day as that's all that's needed by pot weight. I do feed 1/2 gallon at full strength once per week. The rest of the time I water with 4 ml. of Cal Mag added per gal. @ 6.4 pH.

OK, decided to flush since no replies that answered the question. Used 4 gal. of 6.4 pH water & 1 gal. of 6.4 pH water with 10 ml. of Cal Mag added for the final gallon. So everything was 6.4 going in & came out at 6.1. That should be in a decent range for sure now so I can eliminate that as a possibility. If leaves still keep yellowing I'll guess it's either root bound or it's just going through it's normal flowering cycle for this strain.