Red stems on fan leaves and some main branches.

No diary, no pictures showing the issues........hard to give great answers. If you are this close to harvest, it is doubtful that anything you do will solve/fix any issues before you chop it down. Keep it healthy and continue as planned. There is no need to drown your plants with gallons of water as a "flush", all that is required is to stop all fertilizers/nutrients/supplements for the last 10-14 days before harvest and only give plain water in normal amounts and at normal intervals, and the plant will flush itself naturally. If you have given huge amounts of water, this can release built up nutrient salts from the substrate and is most likely the reason for your current problems.

Was there some magical product you added to improve "nutirent flush?" If so, stop using that stuff. FF feed charts have you flush occasionally because they instruct you to overfeed. This is merely to make you buy their fertilizer more frequently than necessary. Giving water-only, especially with excessive runnoff, wouldn't cause an overnight "overfeeding" symptom, so i'd wager it's either some nonsense additive to "improve" a flush or you didn't need a flush and caused some minor deficiencies stripping significant amounts of nutrients from the soil. It's hard to be certain with so little information. when it comes to something as drastic as flushing lots of extra water through the substrate, don't do such a thing on a whim. Do it because there's a major toxicity in the plant that you somehow ignored for a long period of time and you want to 'reset' the substrate so it doesn't pile on. Once a plant has too much of this or that nutrient, it just takes time to use up that excess of building blocks. don't think flushing reduces anything inside the plant, it merely stops the overfeeding, but what was overfed still has to be worked off, not unlike fat in a loose parallel. And, it seems in this case, it did nothing but cause issues for you and potentially with no reason to do it at all. Beware bruh-science and bad feeding instructions.

Flushing is big myth. Some swear by it, science says oposite ;) Overwatered or first time dissolved build up of nutes. Keep on or stop asap. Results cant be predicted, anything from bud rot to starved but tasty plants is possible.