What Could Be The Cause For Increased Hair Fall, Dandruff And Dry Scalp?

I’m 21 years old. I have just recently been noticing thinning of my hair. I have never cut it, nor do I plan to. I have always had decently long hair, definitely hitting below my butt. Now it is barely at the end of my back. I don’t know what the cause may be. I’m young, the only thing that I can think of is stress. I used some shampoo that was supposed to increase hair growth and it ended up causing clumps and large amounts of long hair strands to fall out. Ever since this happened (about 3 months ago) I consistently have fallout in the shower and in my hair brush. I don’t know if some shorter hair in the back is broken off, or new hair growing in. I’m using aveda Invati advanced, which is supposed to reduce hair loss. I also have very bad dandruff and dry scalp. I just need some advice.