Sudden Change in Hair (for the worst)

My name is Nazz, I have currently joined the ABC Homeopathy, and I have a serious concern I would like to address.

I reside in Canada, am 18 years old, and have always had a VERY thick, full, lush head of hair. They were so thick that even if my hair was drenched and wet, you could not see my scalp. I have never done anything to my hair (absolutely no dye, perming/styling, gels, mousse, not even blow drying).

I have a fairly healthy diet (eat a mixture of everything) and also take 3 capsules of omega-3 fish oils daily.

A few months ago, I bought this block of so-called 'natural' henna and used it on my hair.. It was from the company called LUSH (if you would like details/ingredients of this henna, Google in: 'Lush Henna Caca Brun').

Anyhow, over time, I have noticed that my hair has thinned, dried out, its brittle, sheds a lot, and its volume and thickness have been significantly reduced. Its not the hair I once had.
Also, my I've felt my scalp no longer produces that 'healthy' sebum which would lightly coat the hair shaft so it would not tug when I brushed it.

I have not changed anything in my diet, or used anything else in my hair.

Could this be the result of the henna? Because although many people make claims that henna is natural and its good for you, some say that it dries out the hair over
time, makes it brittle and weakens it.

Other than the henna, we have also had a pretty brutal winter here..and often I would not have my hair covered.

Could the effects of the cold weather (snow, chilly winds, freezing rain) and the henna have somehow had a combined negative effect on my hair?

I don't know what to do. I'm so worried. I've read many reviews about Arnica Montana, but I wanted to post a new thread and give background info on my situation.

I would greatly appreciate any help. Thank you very much for taking the time to read this.

Eagerly awaiting a reply.