I don't see how longer hair = dry scalp. That makes no sense. I'd either look for a different shampoo, something to moisturize your scalp, or see a dermatologist as suggested above.
As far as hair falling out, one can lose about 50 to 100 hairs per day which you might think is a lot but is probably normal. If you start seeing thinner patches or sections then you're probably experiencing male pattern baldness.