Feeling exhausted

Hi
I have lost 16 lbs over the last 3 months. I almost entirely removed sugar/fat, introduced oatmeal and raw veggies each day. I work out on my elliptical 1 hour per day for 5 days a week, followed by 10 mins of light yoga. I am finally losing weight. But I am exhausted despite really healthy lacto vegetarian diet. Am also pre-menopausal but that cannot be it. I know I need to do short bursts of exercise - how much? Also I am sleeping a lot now, but still tired. Help!