Help Please!

After about 6 months of dieting, my hair is seriously thinning at the top and is sore/tender in those areas. I'm 23 and have always had thin hair, but it grew somewhat fast. It hardly grows now and seems to still be thinning, even though not as much. I take a multivitamin, and have stopped dieting, but I don't know what to do get it growing again. Also, I get a perm, not a relaxer, and am scared to get another one.

Can anyone help?

I've heard and have read that nutrition plays a big part in hair, but I can never seem to find out exactly what I'm supposed to eat or vitamins to take, so it makes it hard. But before, I would eat mostly vegetables & some chicken or tuna, but my caloric intake was very low. Now, I average about 1800 calories a day & eat carbs, vegetables, fruit, and still pretty much just chicken and fish. Do you know what foods are good for hair? Also, I've heard that rubbing castor oil on the head is good, do you know if this is true?

Thanks.