it's most likely a nitrogen def., A lot of times when you switch to flower and immediately change your ferts over to phosphorus heavy bloom nutes, you aren't getting enough nitrogen. Changing over to flower will cause the plants to stretch again like they first did in veg. Too much nitrogen later in flower can definately be a problem and effect your finished product, but at the same time, having enough nitrogen the first 3 or 4 weeks of flower through the stretch is also crucial. There is also a possibility you aren't feeding enough, but if the plants were fine up until you began flowering and have steadily worsened since you began using flower nutes, it's more than likely a deficiency.
N - NITROGEN (N)
Pale plants, red stems, smaller growth. Rapid yellowing of lower leaves progressing up the plant. Add any chemical fertilizer containing N. Treated plants recover in about a week.
The other possibility could be the beginning of root rot. How are your roots looking? if they are white and healthy looking, they are probably good. If they are starting to turn more brownish tinted and getting slimy, you need a little hydrogen peroxide and to clean out the system.
