First I want to say that I hope you get to feeling better soon.
I have had hypothroidism for about 10 years.(had symptoms for years and kept being told I didn't have it, finally went to a doctor who made a proper diagnosis) I only started taking medication about 3 years ago. Funny thing is once I started the medication, I put on more and more weight. Odd.. I know. It was only recently that my dosage was increased to a point that my levels were in the norm range for me(based on how I feel, etc).
My advise would be to let your doctor know about your weight gain and symptoms. It's also important to get your bloodwork done every three months until you get on the correct dosage.
I agree with raebeaR, in my case I knew I just didn't feel right and had to stay on top of it with my doctor until I did.