I lost my hair after coming off the pill (Dianette). It came out in handfuls. I used to have a crazily thick blanket of hair that reached my butt, then suddenly it was gone and I had to have it all hacked off. Not to sound like too much of a superficial bimbo, but I think I had a mini nervous breakdown about it!
It's grown back a bit, just not the same thickness anymore so I can't keep it as long as I'd like to or it just looks straggly and awful - still miss it terribly, I used to use it to hide behind, and I feel like my armour's gone. So I know how you feel. It's bloody awful. The condition's known as telogen effluvium, I believe, and can happen after stress or a shock to the system, whether that's physical or mental. Usually the hair loss will start 2-3 months after the 'shock.' Your hair follicles are in varying states of growth or rest, and the shock can send a bunch of them into rest (telogen) and a few months later they fall out. Has this started around 2-3 months after any particular big stress in your life? Or would that timeline fit with when you started the diet or lowered calories significantly?
I really wouldn't try going under 1200 calories a day for any prolonged period of time. However varied your diet, it's still so restrictive in terms of sheer mass that your body can go into starvation mode and that's when the body decides it has bigger things to worry about than hair, because it's putting its main energies into just keeping you going. If you're under 1200 at the moment, I'd seriously think about eating more - especially good fats - oily fish, avocado, nuts, just to bring you up to 1200-1300.
The hair loss could be a hormonal problem, or it might be a mix of hormones and diet, or diet affecting hormones. You could be deficient in iron, could be thyroid-related. Would be worth having a chat with your GP if it continues. You don't suffer from pcos, do you?
I know it's horrible *hug*. One tip I have just for dealing with the thinness is to have a look on ebay for clip in extensions. There's some great human hair ones - you should be able to get an 8 piece pack for around 30 quid, and you really wouldn't have a clue it's not your own hair if you clip them in carefully. They add a load of body while your hair's growing back!