It could be one or several things. Here are some things you need to consider. Is it very dry in your home? Has the weather gotten very cold where you live? Are you under new stress? Taking new medicine? Stop taking meds? Change your shampoo or creme rinse? Did you purchase a new blow dryer?
I don't recommend the Head and Shoulders as it dries your hair our. I have longish thick red hair and I use Moisturizing vanilla shampoo and creme rinse that you have to order online from bath and body works. Other than that I highly suggest Fruitis for dry hair and the conditioner it really works. Plus it makes your hair so shiny.
I hope I may have helped you, let me know okay.
Sometimes changes in diet can cause or increase dandruff. Have you changed what you've been eating lately?