I echo many of the above responses about thryroid - am headed to my doc today, in fact, because my worst symptom of hypothyroidism and extremely low iron is hair loss, and I'm again in a bad cycle.

I want to just say that this is one of the most underdiagosed and ignored issue for women - very rigid, conservative (often male, but I've seen a boatload of women who have been this way) physicians just completely dismiss women who complain of weight gain.
I have found a caring, wholistic internist and there is a trend toward this type of doctor for many things, including for the very specific treatment of thyroid and hormone issues.
Google "Mary Shomon" - she is a crusader for appropriate care for this condition and there is a lot of good information and advice.

No matter what your situation is, the smartest thing is to be your own best advocate, and get educated. Which is really hard when we feel crappy/fat/tired/stressed/unattractive/ill.

Good luck!!!