Celexa and sadness

I just started Celexa 2 weeks ago, and so far it is making me very tired. I am taking it for pms symptoms like irritability and anxiety. The first night I took celexa, it made me very sick. I had terrible diarrhea, I was sweating, extremely dizzy, and finally spent about 2 hours vomitting. My doctor then said to take only 1/2 of a pill (10mg) for 3 days with food, and then take the whole pill (20mg) after several days if I can tolerate the 10mg. So now I have been taking the 20mg at night for about 2 weeks, and I haven't felt anything different except that I'm extremely tired all the time. I could literally sleep for 16 hours a day if I could. I usually run in the mornings, but I am too tired for any exercise. I just have no energy, Will this get any better? If I don't get my energy back in the next couple weeks, I am going to stop taking it.