I'm 44 and haven't had a period for 17 months now. I don't think my doctor believed I was post menopausal when I went to see him last week so he ordered blood tests to check hormone levels. I went back for the results on Tuesday and he agreed that I needed HRT to combat my symptoms of hot flushes, night sweats, moodiness, weight gain that won't shift and various other niggles. He said a lot of women gain weight around the middle at this time of life and it can be hell to shift.
Another reason is hypothyroidism.
It wouldn't hurt to ask your doctor to test for both. :)