This is completely normal for some strains and quite common when flowering.
It is thought that growing narrow leaves in flowering makes it easier for pollen to "find" the femalw flower pistils and aid in fertilization.
It is not a nutrient, pH, temperature, humigity or lighting issue, just a normal occurence and nothing to worry about.
I have grown dozens and dozens of plants that have done this exact thing when flowering and have come to expect it.
Your plant is fine, there are no issues to be concerned over regarding the narrow leaves in flowering, it is a genetic function.
Hope this helps, Organoman.